Add API to update workspace folders

[bpasero]

fixes #35407

[wangpung12]

Good

[bpasero]

@jrieken ready for some feedback. as discussed the WorkspaceFolder is now a live object (even for the case of the normal $acceptWorkspaceData). On top of that, there is now an unconfirmed workspace that is used for all extension host manners until $acceptWorkspaceData is called. Only for the events we actually use the _confirmedWorkspace to be able to compute the true events from the previous confirmed state.

[bpasero]

@jrieken I added some tests. fyi I noticed that none of the onDidChangeWorkspace reported a failure when the assertion fails. I think errors in event handlers are silently ignored unless you pass them out to the done handler.