Implement asynchronous tooltips

[colin-grant-work]

What it does

Closes #16362 by adding asynchronous provision to the status bar and hover systems.

How to test

1. Implement a VSCode plugin that runs code like the following (I added it to sample-plugins/sample-namespace/plugin-a/extension.js and symlinked that into the plugins folder. Then you can trigger the code by running the 'hello from plugin a' command):

    const myStatusItem = vscode.window.createStatusBarItem(vscode.StatusBarAlignment.Left);
    myStatusItem.text = 'See what happens!';
    myStatusItem.tooltip2 = async (token) => {
        await new Promise(resolve => setTimeout(resolve, 5000));
        if (token.isCancellationRequested) {
            throw new vscode.CancellationError();
        }
        return 'You waited long enough!';
    }
    myStatusItem.show();

2. The status item should appear.
3. You should be able to hover it.
4. Initially, a Loading... message will appear.
5. After a few seconds, the message 'You waited long enough!' should appear.
6. If you move away from the item, it should cancel the hover, and it should not appear or interrupt another hover after the time has elapsed.

Follow-ups

Breaking changes

• This PR introduces breaking changes and requires careful review. If yes, the breaking changes section in the changelog has been updated.

Attribution

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

[ndoschek]

Thanks @colin-grant-work!
Code looks great to me, and testing with the minimal extension worked as expected!
The code looks great, and testing with the minimal extension worked as expected 👍
This is a really nice improvement looking at the new features the upcoming git extension version will bring 👍

[ndoschek]

nit: Maybe add a comment here, as tooltip === true is not obvious on the first glance to everyone.