- Adds annotations to return types and parameters that can be undefined
- Add conditional guards
- Converting a few `return undefined` to `return false`. Based on how these values are used, I don't think this should change the behavior

The fact that `createSnapshot` can return null is pretty annoying. One fix would be to use `IResolvedTextFileEditorModel` in more places in our code base, but I am not confident in making this change.

* adopt in tests

* adopt more join() that are straight forward

* more safe usages

* more adoption

part of strict null work

Throw not implemented instead of returning null results for most methods

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

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