In MonacoEditorModel, use value for snapshot

[colin-grant-work]

What it does

Fixes #12406 by changing the snapshotting logic in MonacoEditorModel

It turns out that the brand of ITextModel that we use in Monaco has this size restriction on its snapshots:

https://github.com/microsoft/vscode/blob/7bd1368261677fa160c28ff043c444d905331ee7/src/vs/editor/common/model/textModel.ts#L155-L157

That means that in general, we want to use the value rather than delegating to that implementation of createSnapshot.

How to test

1. Create a large file (> 64kb)
2. Use save as to create a copy of that file.
3. Check that the new file has the same size and content as the original.
4. Create a large untitled file.
5. Use save (as) to save it.
6. Ensure that the new file has the same size and content as its untitled source.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[vince-fugnitto]

LGTM 👍
I confirmed that the size after performing a save as... is the same as the original.