Refactor key dispatch

[ConradIrwin]

Simplify key dispatch code.

Previously we would maintain a cache of key matchers for each context that
would store the pending input. For the last while we've also stored the
typed prefix on the window. This is redundant, we only need one copy, so now
it's just stored on the window, which lets us avoid the boilerplate of keeping
all the matchers in sync.

This stops us from losing multikey bindings when the context on a node changes
(#11009) (though we still interrupt multikey bindings if the focus changes).

While in the code, I fixed up a few other things with multi-key bindings that
were causing problems:

Previously we assumed that all multi-key bindings took precedence over any
single-key binding, now this is done such that if a user binds a single-key
binding, it will take precedence over all system-defined multi-key bindings
(irrespective of the depth in the context tree). This was a common cause of
confusion for new users trying to bind to cmd-k or ctrl-w in vim mode
(#13543).

Previously after a pending multi-key keystroke failed to match, we would drop
the prefix if it was an input event. Now we correctly replay it (#14725).

Release Notes:

• Fixed multi-key shortcuts not working across completion menu changes (#11009)
• Fixed multi-key shortcuts discarding earlier input (#14445)
• vim: Fixed jk binding preventing you from repeating j (#14725)
• vim: Fixed escape in normal mode to also clear the selected register. (#14311)
• Fixed key maps so user-defined mappings take precedence over builtin multi-key mappings (#13543)
• Fixed a bug where overridden shortcuts would still show in the Command Palette