SCM Working Set - persist pinned tabs when switching branches

[trevordunn]

I love the new Working Sets feature (#35307) - nice work! In particular I like setting mine to "scm.workingSets.default": "empty" so I have a fresh slate when switching branches, though it'd be nice if there was a way to persist the pinned tabs when switching branches since the files I have there tend to be ones I need to reference across branches.

I see two approaches to this, in order of preference:

1. Add another setting named something like "scm.workingSets.persistPins": true where pinned tabs are maintained globally across working sets. If you close/open a pinned tab in one working set, it will remain closed/opened when switching working sets.
2. Add a third scm.workingSets.default option named something like pinned. This would carry forward just the pinned tabs I have in the current working set if the switched branch doesn't have a working set yet. I don't like this one as much because it means working sets won't always have the same set of pinned tabs when I add/remove pins.