romgrk
Add resession extension support
Adds an extension for https://github.com/stevearc/resession.nvim to session save and session restore using resession. Fixes resession bug https://github.com/stevearc/resession.nvim/issues/63 and is also (imo) generally a bit nicer of a solution than using globals. Ideally this becomes the "most advisable" way of doing session management using barbar - this seems to work pretty damn flawlessly in my testing, having spent a while using persisted.nvim.
Note that the code is mostly based off of how barbar was already doing session save and session restore.
I've made some changes in the second commit to make it slightly cleaner IMO, but this behavior is slightly less aligned with the way that barbar does session restore using globals. Let me know what you think is better.
