Timeline providers that don't need URI's should be able to render on empty workbench

#84297 Currently when the workbench has no ActiveTextEditor, the timeline view won't load anything. But there is a category of "workspace timeline" events that should be able to work without any active editor. For instance: Timeline of past/future sprints/iterations Timeline counting down days till code freeze/GA/etc. Timeline of git tags/releases I think the provideTimeline should give an optional uri, and if an extension needs the URI they simply return undefined to be excluded from the timeline. But if not, they can elect to be shown even when theres no active editor.