Resolves #166314

These changes are a proof-of-concept and are not complete, but my hope is that putting them up can start a discussion about how they might get reviewed and potentially accepted.

Current limitations:

• "current editor" and "default" history stacks are completely separate, which leads to some slightly sketchy behavior. If you have changes A, B and C in the current editor, and then run "go back in edit locations" twice your cursor will be at A. If you then run "go back in edit locations in current editor", your cursor will be at B.
• I have made no attempt to update the existing context keys scheme. I think there's a discussion to be had about which context keys might make sense.
• In the linked issue, I discussed unifying all the navigation commands into a single command that takes scope and filter arguments. I have made no attempt to make that change, since that's somewhat orthogonal to the underlying history service functionality. If that change is desired, I'm happy to add it to this or another pull request.
• I haven't added or updated any tests. I figured let's decide what the changes are first, then I can write tests that verify the behavior.