Migrate viewlet-level progress indication to view-level

[eamodio]

Refs: #92135

Now that we can support view-level progress indication, there doesn't seem like much reason to have viewlet-level progress.

This can also address the concerns here: #92136 (review)

/cc @bpasero @sbatten

[bpasero]

@eamodio I guess one problem is that extensions can still use API to report progress on the viewlet container right? I forgot about that when I suggested this, so we may not easily be able to remove this.

[eamodio]

Yeah, we won't be able to remove it, but we could deprecate it to encourage view-level use.

I've added an API proposal here for extensions: #92421

[eamodio]

Since we really only expose ProgressLocation.SourceControl (and it is kind of special), I don't think its worth doing anything here.