Allow to remove watch expressions one by one

[msujew]

What it does

I noticed in #11953 that watch expressions can only be removed in bulk. This change aligns the UX to vscode, where users are able to remove watched expressions one by one.

Also fixes an existing issue where watched expressions which evaluate to an object result in an overflow:

How to test

1. Start debugging a Test in Theia
2. Add multiple watched expressions
3. Confirm that they can be removed one by one

I believe the theia-tree-element-node css class is reused in the debug console to display evaluated expressions. Assert that no regressions appeared due to my changes.

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]

@msujew I believe we might want to improve the styling by:

• only showing the remove expression icon on hover (similarly to vscode)
• aligning the icon with the row

[msujew]

@vince-fugnitto Thanks, didn't notice these two differences. Funnily enough, vscode's button also isn't correctly vertically centered with the row :D

[vince-fugnitto]

The changes look good to me, and I confirm that we can now discard individual watch expressions 👍