Code like:

```
private readonly _onDidEncounterLanguage: Emitter<LanguageId> = new Emitter<LanguageId>();
```

Does not need to repeat the `Emitter<LanguageId>` type annotation

This reverts commit 677234a7.

#60565

- avoid public modifier
- use Disposable where applicable
- fix some event handler leaks
- clean up some TODO@ben