[vscode][proposed] Stub onWillCreateEditSessionIdentity

[marcdumais-work]

What it does

This event was recently added to proposed API EditSessionIdentityProvider. For now we add a stub that will allow vscode.git@1.77.0 and more recent to activate and be successfully used in Theia applications.

Fixes #12521

How to test

You can use the vscode.git 1.77.0 extensions below:
git-.zip

• Extract the git extensions and copy them in the "plugins" folder.
• start the example application, open a workspace
• vscode.git should activate, not complaining about onWillCreateEditSessionIdentity not existing. Built-in git will not ultimately work, unless @theia/git is removed from the app - but that's no necessary for this PR)

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]

Woudn't the simplest stub implementation just be an empty function function? No need for emitters and such.

[vince-fugnitto]

LGTM 👍
I confirmed that the builtin activates as expected, and the error is not thrown:

Activating extension 'Git (built-in)' failed: r.workspace.onWillCreateEditSessionIdentity is not a function

[marcdumais-work]

Woudn't the simplest stub implementation just be an empty function function? No need for emitters and such.

That's correct. I went a bit further than the simplest. I would not mind going with something simpler.

[marcdumais-work]

@tsmaeder as discussed I updated the 1st commit to use a separate proposed.d.ts file instead of adding to theia-proposed.d.ts. I took a fresh copy from vscode@1.77.0, only slightly modifying it to adapt to our namespace.

Then I added commits on top to simplify the stub and tidy-up.

[marcdumais-work]

I will resolve the conflicts locally after the other proposed API stubs are merged.

[vince-fugnitto]

@marcdumais-work it should be fine to rebase now 👍

[vince-fugnitto]

LGTM 👍