Use cellHandle instead of index when talking to output webview

[dhuebner]

What it does

Now using cellHandle instead of index when "talking" to the output webview. The reason is that in some cases (cell deletion) there is no way to identify a visible cell to the corresponding start index as the notebook no longer contain this cell.

Doing so, we also align cellsSpliced to existing cellMoved webview communication channel, that already uses handles instead of ids.

How to test

Open a notebook with many cells that has output. Delete some cell in the middle. Check that there are any old/overlapping outputs.

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

[dhuebner]

@JonasHelming
How to create a next release from this PR? This is an important fix...

[jonah-iden]

@dhuebner for a next release you just go to actions in this tab. Select the publish next release action and then run the run workflow button. There you can select this branch.
It could be that it goes red but still publishes all important packages

[jbicker]

LGTM