Add CI workflow for tests and package build

Run tests across 3 OSes x 3 Python versions on push to main
and on pull requests. Includes package build verification.

Author: JSv4

.github/workflows/ci.yml

@@ -0,0 +1,75 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+permissions:
+  contents: read
+
+jobs:
+  test:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, windows-latest, macos-latest]
+        python-version: ['3.9', '3.11', '3.12']
+
+    steps:
+    - uses: actions/checkout@v4
+      with:
+        submodules: recursive
+
+    - name: Set up Python ${{ matrix.python-version }}
+      uses: actions/setup-python@v5
+      with:
+        python-version: ${{ matrix.python-version }}
+
+    - name: Set up .NET
+      uses: actions/setup-dotnet@v4
+      with:
+        dotnet-version: '8.0.x'
+
+    - name: Install dependencies
+      run: |
+        python -m pip install --upgrade pip
+        pip install hatch
+
+    - name: Build engine binaries
+      run: python build_differ.py
+
+    - name: Run tests
+      run: hatch run test
+
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+      with:
+        submodules: recursive
+
+    - name: Set up Python
+      uses: actions/setup-python@v5
+      with:
+        python-version: '3.11'
+
+    - name: Set up .NET
+      uses: actions/setup-dotnet@v4
+      with:
+        dotnet-version: '8.0.x'
+
+    - name: Install build dependencies
+      run: |
+        python -m pip install --upgrade pip
+        pip install hatch hatchling
+
+    - name: Build package
+      run: hatch build
+
+    - name: Check package
+      run: |
+        pip install twine
+        twine check dist/*