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/input/multitap_advanced_example.dart

@@ -56,7 +56,8 @@ class MultitapAdvancedExample extends FlameGame
 
   @override
   void onDragEnd(int pointerId, _) {
-    final start = this.start, end = this.end;
+    final start = this.start;
+    final end = this.end;
     if (start != null && end != null) {
       panRect = start.toPositionedRect(end - start);
     }