Replaces many loops of the form:

```js
for (let i = 0; i < elements.length; ++i) {
    const i = elements[i];
   ...
}
```

with:

```js
for (const element of elements) {
    ...
}
```

Mix of a horrible regex based find/replace and manual touch ups

Part of #58167

* 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 c9efee46.

Use `Array<T | U>` when dealing with union types instead of  `(T | U)[]`

Many `Promise.resolve(null)` should actually be `Promise.resolve(void 0)`. In other cases, we were missing a null return annotation.