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
```

packages/flame_isolate/example/lib/brains/worker_overmind.dart

@@ -43,9 +43,9 @@ class WorkerOvermind extends Component
       _queuedTasks.add(Pair(objectToMove, destination));
 
   @override
-  void update(double t) {
-    _assignTaskInterval.update(t);
-    super.update(t);
+  void update(double dt) {
+    _assignTaskInterval.update(dt);
+    super.update(dt);
   }
 
   /// Set that keeps track of what workers are currently in queue to get a job.