feat: AppLocalizations CodeLens preview in Dart code

[Zensonaton]

What is it?

This PR introduces CodeLens support for Dart files that use AppLocalizations class. The feature can be configured via VS Code settings:

How it works

The current algorithm scans Dart code for usages of AppLocalizations. When it infers that a given symbol (e.g., a variable) might be an instance of the AppLocalizations class, it issues a Dart LSP request to verify the actual type.

Notes

1. It’s unclear whether this feature should be enabled by default. You can easily disable it by changing the default value in package.json.
2. The feature might introduce a slight performance overrhead. In practice, no issues were observed during testing on a real Flutter project, since CodeLens is only computed for the currently open file.

Future ideas

1. Clicking a CodeLens currently has no effect. A reasonable next step would be to open the corresponding .arb file on click.
2. Refactoring support from the Dart side. For example, renaming AppLocalizations.helloWorld could propagate the change to the helloWorld key across all .arb files.
3. "Go to Implementation" in Dart code (Ctrl + Click) should navigate to the relevant .arb entry instead of AppLocalizations.dart.
4. "Go to Implementation" in .arb files should navigate to the template .arb file.
5. "Go to References" in .arb files (Shift + F12) should show all Dart code usages.