pub.dev score: license, dartdoc, analysis, tests

- Add MIT license
- Add dartdoc to all public APIs (UxKeyboard, BendBox, Bezier, Json)
- Add analysis_options.yaml with flutter_lints and public_member_api_docs
- Add repository, issue_tracker, topics to pubspec.yaml
- Expand package description
- Fix BendBox constructor (const, super.key)
- Remove unused import in keyboard.dart
- Replace boilerplate tests with UxKeyboard smoke tests
- Move ux dependency to example's dependencies (not dev_dependencies)

test/ux_test.dart

@@ -1,21 +1,16 @@
-import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:ux/ux.dart';
 
 void main() {
-  const MethodChannel channel = MethodChannel('ux');
-
-  setUp(() {
-    channel.setMockMethodCallHandler((MethodCall methodCall) async {
-      return '42';
-    });
+  test('UxKeyboard.instance is a singleton', () {
+    expect(UxKeyboard.instance, same(UxKeyboard.instance));
   });
 
-  tearDown(() {
-    channel.setMockMethodCallHandler(null);
+  test('UxKeyboard.height starts at 0', () {
+    expect(UxKeyboard.instance.height, 0);
   });
 
-  test('getPlatformVersion', () async {
-    expect(await UX.platformVersion, '42');
+  test('UxKeyboard.isOpen is false when height is 0', () {
+    expect(UxKeyboard.instance.isOpen, false);
   });
 }