A few small improvements for the key bindings modal

[acao]

We could add a little footnote on the keybindings dialog if you want that contains a little link to these docs, keyed by prop.keyMap:

https://codemirror.net/5/demo/sublime.html
https://codemirror.net/5/demo/vim.html
https://codemirror.net/5/demo/vim.html

we should also add docs for this, or backport a way to programatically dynamically (and thus optionally ofc) import the additional vim and emacs modes

also, just noticed i was completely wrong about that vim PR! this does have insert mode already, i forgot, this should be good to say we already have vim support. i will correct myself there.

[thomasheyenbrock]

I added this to the short key dialog in #2601. Regarding dynamically importing the CM keymaps, there is #2583, so I'm closing this issue as half completed and half duplicate 😉