Repeat Key: Overhaul implementation.

This commit substantially revises the implementation of Repeat Key to
leverage keyrecord_t's .keycode field instead of setting the keypos
for the intended key. The advantages of this are that

* This eliminates the complication of temporarily setting the last
  layer state.

* The keypos can now be set to the Repeat Key that's being pressed, as
  it should be, which is better for e.g. reactive RGB matrix animations.

* We can now *set* the keycode to be repeated to any keycode, even those
  that are not necessarily mapped in the layout.

* Most significantly, we can also use this same approach for Reverse
  Repeat, so that implementations and interaction with
  `process_record_user()` are analogous. The consistency makes it
  simpler to think about.

The one minor downside is that it is now required that the Combos
feature is enabled to use Repeat Key, since the critical .keycode field
is only present when Combos are enabled.

Additionally, I refactored the reverse key definitions to eliminate the
need for defining a big table in keymap.c. Instead, a sensible set of
reverse key definitions are used by default, and these may be added to
or overridden by defining a `get_rev_repeat_key_keycode_user()`
callback.

Please see the updated documentation for full details:
<https://getreuer.info/posts/keyboards/repeat-key>

Author: getreuer