Apply Flutter best practices [Non-Breaking] (#2)

* Add flutter_lints package

* Use key in widget constructors

lint: use_key_in_widget_constructors

* Unnecessary new keyword

lint: unnecessary_new

* Prefer const with constant constructors

* Exclude protobuf files from analyzer

* Private field could be final

* Annotate overridden members

* Use initializing formals when possible

* Don't access members with `this` unless avoiding shadowing

* Prefer is! operator

* Use isEmpty instead of length

* Use collection literals when possible

* Revert comment changes

* Cleaner null testing

* Misc fixes

* Avoid using `forEach` with a function literal

* Prefer `final` over `var` where applicable

* Enforce stricter type-checking

* Ignore VS Code files

* Flutter format

* Prefer using lowerCamelCase for constant names

* flutter format -l 100

* Disable `avoid_print` for examples

* Slight improvements to example

Should solve lint error: no_logic_in_create_state

lib/src/widget/video_track_renderer.dart

@@ -45,20 +45,20 @@ class _VideoTrackRendererState extends State<VideoTrackRenderer> {
     super.didUpdateWidget(oldWidget);
   }
 
-  _trackChanged() {
+  void _trackChanged() {
     setState(() {
       _renderer.srcObject = widget.track.mediaStream;
     });
   }
 
-  _initRenderer() async {
+  void _initRenderer() async {
     await _renderer.initialize();
     _trackChanged();
   }
 
   @override
   Widget build(BuildContext context) {
-    var isLocal = widget.track is LocalVideoTrack;
+    final isLocal = widget.track is LocalVideoTrack;
     return RTCVideoView(
       _renderer,
       mirror: isLocal,