Use VS Code API to contribute view to the secondary sidebar

VS Code PM here 👋

VS Code now allows extensions to contribute views to the secondary sidebar
https://github.com/microsoft/vscode/issues/151681

Keep in mind that this API is still in proposed state, and we aim to finalize it start of October. So it would be great if you try out the API - let us know if it works for you, and once it is final you should be able to use it.

Right now you use a custom editor on the side, and it might be nicer if you have an actual view in the secondary sidebar (layout functionality will be nicer).

cc @igorkofman