feat: history buffer

[benlubas]

closes #138

Currently MVP. There's a split history window that shows the history of the current cell when you open it.

TODO:

• Auto Update
• Images with autoupdate... it seems to work for the first few runs, and then starts breaking horribly
• Okay, this one isn't really done. It still causes rendering issues after running a cell that produces an image too many times after the history buffer is created.
• [ ] Ability to show global history
• [ ] Timestamps?
• history window images should get separate image IDs to the virtual text image in the buffer, otherwise they will disappear (maybe only if auto update isn't on).
• clear images on buffer close (calling history while buffer is open already will break things right now
• README, and other doc updates

I think that I'm going to scrap the ability to view this in a floating window. Doesn't seem that useful to me.

Also pushing time stamps and global history to later PRs b/c this one is already really big.

[benlubas]

the problem with images is just a problem in general with running things while splits are open.

[benlubas]

I think I'm dealing with an image.nvim bug here. when the history buffer exists (even if it's not open), image rendering breaks.

The reason I think this is an image.nvim issue, is that causing a re-render (either by folding and unfolding or by unfocusing and refocusing the editor) will render the images like normal.

[benlubas]

I think this will have to wait for the image.nvim rewrite..

Hopefully that will fix a lot of the bugs that this PR is dealing with.

[benlubas]

The latest image.nvim change fixed the worst of these problems. There are still some issues with images unlinking themselves and hanging around though.