Strict null check textFileService

- 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.

- Avoid leaking implementation details of hasAutoFix
- Better encapsulation

Fixes #70113

Fixes #70194

Fixes #68896

Fixes #70229

Investigating why these changes were required. Adding workarounds until then

Use TS's updateOpen api to batch file changes

For #64485

Batching file changes should be more efficent than sending requests one at a time.

Strict null check work in extHost and mainThread language Features

- annotate undefined returned types
- Don't call functions with null or undefined values.
- Refactor _convertCompletionItem so that we don't need to mutate the returned object
- Suppress a type error related to electricCharacters.