* strictNullChecks for macLinuxKeyboardMapper

* Fix strict null check for assertResolveUserBinding

* Add comma

Better strategy for #62409

This reverts commit 5897b877.

Continues #60565 for converting tests. A few files used TypeStat to add `!`s but the following interface changes were manually inferred from implementing classes:
* `IActiveCodeEditor`'s methods can all return `null`
* `IEditor` needs to report `getControl(): IEdotorControl | null` because `BaseEditor`'s `getControl` can return `null`

* Converted another ~70 test files to strictNullChecks

These are the remaining test files that didn't require any changes to non-test file types. I'll send those in separately.

Most changes were automatic and small (`!`s).
The only non-trival change is that `extHostSearch.ts` needs an interface for its test data.

Again used [TypeStat](https://github.com/JoshuaKGoldberg/TypeStat) when possible per #65581. I'm happy to split this up into smaller PRs if you'd prefer!

* Added missing `!`s to `commands.test.ts`

* Manually added strict null checks in around 70 files

* Fixed tsconfig.strictNullChecks.json merge fail

* Touched up a few unnecessary type changes

* Removed files already in other PRs

This reverts commit 85eb3a1f.

Code like:

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

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