Complete library (#3)

Transform the project into a package and reformat files.



Full Changelog:

* Used package template (flutter create -t package .)
Made untis.dart a library
Fixed warning about not returning value

* Fixed missing color fields from request being put in Subject object

* Removed unused fore and backColor values from Subject

* Reformatted file

* Reformatted file
Improved readability of code
Corrected some variable spelling mistakes

* Reformatted file

* Updated README.md to encourage package management

* Lowered sdk requirements
Upped http version

* Changed line length to 120 symbols and reformatted file

* Update README.md

Fixed small error

* Revert formatting of README.md

* Revert git:// package url to https:// in README.md

The package URL was intentionally prefixed with https:// but was changed to git:// when reverting the formatting changes. This commit re-applies the https:// prefix.

---------

Co-authored-by: randommodder <andreas.powslowsky@gmail.com>
Co-authored-by: Simon Conrad <simon2004.sc@gmail.com>

Author: 3 people

README.md

@@ -5,10 +5,12 @@ An asynchrous WebUntis API wrapper written in Dart.
 Attention: Each method must be properly awaited in an asynchronous fashion. This rather annoying circumstance is inherited by the usage of the http package, but should
 help to design fast & responsive UIs.
 
-Starting off, put [untis.dart](untis.dart) into the lib folder of your project and add these two packages to the dependencies of your pubspec.yaml file:
+Starting off, add this package to the dependencies of your pubspec.yaml file:
 ```yaml
-http: ^0.13.4
-string_similarity: ^2.0.0
+dart_webuntis:
+  git:
+    url: https://github.com/IsAvaible/dart-webuntis.git
+    ref: main
 ```
 
 The basic tool used to interact with the API is the Session object.