Narrow typing of Saveable.createSnapshot

[colin-grant-work]

What it does

Fixes: #11031.

In the Monaco uplift, the return of MonacoEditorModel.createSnapshot was changed, but its effect on the Saveable system was not detected because the return of Saveable.createSnapshot was typed as object. This PR narrows that typing so that callers of the function have some idea what to expect, and deviations from those expectations can be caught.

How to test

1. Open two editors, one with an existing file and one Untitled.
2. Use Save as on both editors
3. In both cases, the content should be copied and the old editor closed.
4. No errors should be logged about failures to create text buffers.

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]

I confirmed that the changes successfully fix #11031 👍