Add an experimental "scratchpad pane", for testing

[zadjii-msft]

Summary of the Pull Request

Builds upon #16170. This PR simply adds a singly type of non-terminal pane - a "scratchpad pane". This is literally just a single text box, in a pane. It's on the { "command": "experimental.openScratchpad" } action.

References and Relevant Issues

See: #997

Detailed Description of the Pull Request / Additional comments

I also put it behind velocity so it won't even go into preview while this bakes.

This is really just here to demonstrate that this works, and is viable. The next PR is much more interesting.

Validation Steps Performed

Screenshot below.

other PRs

• #16170
• #16171 <-- you are here
• #16172

[KalleOlaviNiemitalo]

Screenshot below.

I don't see it.

[zadjii-msft]

(lost track of time at EoD Friday, shoulda posted that earlier)

[DHowett]

"can ScratchpadPane be left out of the idl?"