chore: add issue templates (#13)

.github/ISSUE_TEMPLATE/01_bug_report.yml

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+name: 🐞 Bug Report
+description: Create a report to help us improve 
+labels: ["bug", "triage"]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        **Before continuing, have you:**
+          * Tried upgrading to newest version of the Dark Music Concepts, to see if your issue has already been resolved and released?
+          * Checked existing open *and* closed [issues](https://github.com/michaldivis/dark-music-concepts/issues?utf8=%E2%9C%93&q=is%3Aissue), to see if the issue has already been reported?
+          * Tried reproducing your problem in a new isolated project?
+  - type: textarea
+    id: background
+    attributes:
+      label: Description
+      description: Please share a clear and concise description of the problem.
+      placeholder: Description
+    validations:
+      required: true
+  - type: textarea
+    id: repro-steps
+    attributes:
+      label: Reproduction Steps
+      description: |
+        Please include minimal steps to reproduce the problem if possible. E.g.: the smallest possible code snippet; or a small project, with steps to run it.
+        Always include text as text rather than screenshots so code can easily be copied and will show up in searches.
+        Stack Overflow has a great article about [how to create a minimal, reproducible example](https://stackoverflow.com/help/minimal-reproducible-example).
+      placeholder: Minimal Reproduction
+      value: |
+        ```csharp
+        var chord = Chord.Create(Notes.C2, ChordFormulas.DominantSeventh);
+
+        chord.Invert();
+        ```
+    validations:
+      required: true
+  - type: textarea
+    id: expected-behavior
+    attributes:
+      label: Expected behavior
+      description: |
+        Provide a description of the expected behavior.
+      placeholder: Expected behavior
+    validations:
+      required: true
+  - type: textarea
+    id: actual-behavior
+    attributes:
+      label: Actual behavior
+      description: |
+        Provide a description of the actual behavior observed. If applicable please include any error messages or exception stacktraces.
+      placeholder: Actual behavior
+    validations:
+      required: true
+  - type: textarea
+    id: regression
+    attributes:
+      label: Regression?
+      description: |
+        Did this work in a previous release of Dark Music Concepts? If you can try a previous release to find out, that can help us narrow down the problem. If you don't know, that's OK.
+      placeholder: Regression?
+    validations:
+      required: false
+  - type: textarea
+    id: known-workarounds
+    attributes:
+      label: Known Workarounds
+      description: |
+        Please provide a description of any known workarounds.
+      placeholder: Known Workarounds
+    validations:
+      required: false
+  - type: textarea
+    id: configuration
+    attributes:
+      label: Configuration
+      description: |
+        Please provide more information on your .NET configuration:
+          * Which version of Dark Music Concepts are you using?
+          * Which .NET runtime and version are you targeting? E.g. .NET framework 4.7.2 or .NET 6.0.
+      placeholder: Configuration
+    validations:
+      required: false
+  - type: textarea
+    id: other-info
+    attributes:
+      label: Other information
+      description: |
+        If you have an idea where the problem might lie, let us know that here. Please include any pointers to code, relevant changes, or related issues you know of.
+      placeholder: Other information
+    validations:
+      required: false

.github/ISSUE_TEMPLATE/02_feature_proposal.yml

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+name: 💡 Feature Proposal
+description: Propose a new feature or a change to an existing feature
+title: "[Proposal] "
+labels: [feature-proposal]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        We welcome feature proposals! Describe a feature you'd like to add or change.
+  - type: textarea
+    id: background
+    attributes:
+      label: Background and motivation
+      description: Please describe the purpose and value of the feature here.
+      placeholder: Purpose
+    validations:
+      required: true
+  - type: textarea
+    id: feature-proposal
+    attributes:
+      label: Feature Proposal
+      description: |
+        Please provide the specific public API signature diff that you are proposing.
+      placeholder: API declaration (no method bodies)
+      value: |
+        ```C#
+        public class Chord
+        {
+            public Chord ReverseInvert();
+        }
+        ```
+    validations:
+      required: true
+  - type: textarea
+    id: usage
+    attributes:
+      label: Usage
+      description: |
+        Please provide code examples that highlight how the proposed API additions are meant to be consumed. This will help suggest whether the API has the right shape to be functional, performant and usable.
+      placeholder: API usage
+      value: |
+        ```C#
+        var chord = Chord.Create(Notes.C2, ChordFormulas.DominantSeventh);
+
+        chord.ReverseInvert();
+        ```
+    validations:
+      required: true
+  - type: textarea
+    id: risks
+    attributes:
+      label: Risks
+      description: |
+        Please mention any risks that to your knowledge the API proposal might entail, such as breaking changes, performance regressions, etc.
+      placeholder: Risks
+    validations:
+      required: false

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: true