Cannot use RegExp.lastIndex to -1 because it will not find any matches afterwards on Safari (#90063)

Readonly helps with type guards and code comprehension

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

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

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