Add Inlay Hints to Dart code: variable type, parameter name, and more

The Dart SDK and the VSCode extension of Dart are supported with the Inlay hints. The IntelliJ plugin should follow the same.

See:
- https://github.com/dart-lang/sdk/commit/8e2dbc1cdc05d5cbe134332dfcc8c7dc3c53eff2
- https://github.com/dart-lang/sdk/commit/ba479310b3b7c3d2060499c083a97f4b2ad9bb65
- https://github.com/Dart-Code/Dart-Code/issues/4154
- https://github.com/Dart-Code/Dart-Code/issues/4156
- https://github.com/Dart-Code/Dart-Code/issues/4162
- https://youtrack.jetbrains.com/issue/WEB-53819/Add-Inlay-Hints-to-Dart-code-variable-type-parameter-name-and-more.

Start by reviewing the Dart SDK commits and the linked Dart-Code issues to identify the intended inlay-hint behavior. Then inspect the flutter-intellij plugin's existing Dart editor integration and compare it with the IntelliJ inlay-hints APIs. Done means the IntelliJ plugin provides the requested variable-type, parameter-name, and related hints consistently with the referenced Dart tooling.

Written by the indexing model from the issue text.