Support LSP `$/progress` messages

[connernilsen]

LSP supports a $/progress notification, which notifies the client of what it's working on and what it still has left to do when performing an analysis.

This isn't a super high priority thing, but would be cool and nice to have.

Start looking at:

• Transaction::set_subscriber, where we'll probably need to insert a subscriber somewhere during LSP setup.
• Subscriber, which is what we'll need to implement to send messages