[vscode] Add proposed dropMetadata API

[msujew]

What it does

Closes #12632

This PR adds the groundworks for supporting the proposed API. Note that while the API is supported on a type level, our current monaco version isn't able to actually make use of the changes. I've annotated all places in the code with @monaco-uplift so that any contributor can improve on this behavior once we have performed the uplift to ^1.79.0.

How to test

There isn't really anything to test, no behavior should have changed. Confirm that the changes in #12125 still work as expected.

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

[tsmaeder]

@msujew this seems to work fine. The only strange thing when I test according to the instruction in #12125 is this: I get both a new editor with the file I'm dropping and I get the file contents inserted into the editor I dropped into. Is this the desired behavior?

[msujew]

@tsmaeder I assume that this is an untested combination of #12065 and #12125. Since both PRs were submitted at similar times, I assume that the latter one didn't include the former yet, so I didn't notice the strange behavior during testing. And no, this isn't desired behavior. I believe that vscode has a special mode (activated via shift) to change between file dropping and resolving via the DocumentDropEditProvider. We don't seem to do that.

However, fixing that is imo out of scope for this PR.

[tsmaeder]

I've opened #12748

[tsmaeder]

Since the observed weird behavior cannot possibly be due to the changes in this PR, this looks good to me.