This reverts commit 4a461d26.

Revert "null -> undefined"

This reverts commit
30dc021b

TypeScript always assumes that array index access and object index type accesses return a non-null value. This was causing a few strict null issues to be masked where we were returning `array[i] || null` but the return type was not marked as nullable

The fix is to use `Map` types instead as these have the proper types

Readonly helps with type guards and code comprehension

Code like:

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

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

We compile using the alwaysStrict flag so these directives are not needed

This part removes most `use strict` directives that are right after the file header

There are many places in the code that do `import Event, { ...} from '.../event'`.