refactor: Add new lint rules (#2477)

This PR adds the following lint rules to our list:

```
always_put_required_named_parameters_first
avoid_multiple_declarations_per_line
avoid_positional_boolean_parameters
avoid_returning_null_for_void
avoid_returning_this
avoid_unnecessary_containers
enable_null_safety
library_private_types_in_public_api
no_leading_underscores_for_library_prefixes
no_leading_underscores_for_local_identifiers
prefer_null_aware_method_calls
tighten_type_of_initializing_formals
unnecessary_late
use_setters_to_change_properties
```

And these rules were considered, and some changes were made according to
them as a clean-up, but in many places they didn't make sense
(`prefer_asserts_with_message` I would have included, but there were too
many places that needed to be changes):

```
collection_methods_unrelated_type
prefer_asserts_with_message
avoid_renaming_method_parameters
```

examples/lib/stories/components/look_at_example.dart

@@ -83,7 +83,7 @@ class LookAtExample extends FlameGame with TapDetector {
 
   // Just displays some information. No functional contribution to the example.
   void _spawnInfoText() {
-    final _shaded = TextPaint(
+    final shaded = TextPaint(
       style: TextStyle(
         color: BasicPalette.white.color,
         fontSize: 20.0,
@@ -96,7 +96,7 @@ class LookAtExample extends FlameGame with TapDetector {
     add(
       TextComponent(
         text: 'nativeAngle = pi',
-        textRenderer: _shaded,
+        textRenderer: shaded,
         anchor: Anchor.center,
         position: _chopper1.absolutePosition + Vector2(0, -50),
       ),
@@ -105,7 +105,7 @@ class LookAtExample extends FlameGame with TapDetector {
     add(
       TextComponent(
         text: 'nativeAngle = 0',
-        textRenderer: _shaded,
+        textRenderer: shaded,
         anchor: Anchor.center,
         position: _chopper2.absolutePosition + Vector2(0, -50),
       ),