Finalize `tooltip` API for `QuickPickItem`s

[TylerLeonhardt]

Last iteration we introduced tooltips for quickpickitems into Core and as a proposed API:
#174473

The shape of which looks like this:

        export interface QuickPickItem {
		tooltip?: string | MarkdownString;
	}

The ask was from Jupyter Notebooks extension who show a ⚠️ in their quick pick items when something isn't quite as we expect. A tooltip is one way to allow them to show additional information that might not be critical, but is available.

[gjsjohnmurray]

Any reason not to get this finalized?

[TylerLeonhardt]

I think it makes sense... just need to do it.