Feature: Leader Key

[petejohanson]

We should have a leader key behavior that allows you to add custom actions implemented for a certain sequence of keys pressed after the leader key behavior is triggered.

QMK Docs: https://docs.qmk.fm/#/feature_leader_key

I would imagine there would be something like:

&leader

In your keymap, and you would customize the leader behavior registration itself, elsewhere in the keymap, e.g.:

&leader {
  sequence_pi {
    keycodes = <P I>;
    action = <&not_sure_what_goes_here>;
  };

  sequence_foo {
    keycodes = <F O>;
    action = <&whatever_it_does>;
};

[arabshapt]

This would be awesome! I am not sure how hard it would be to implement it... I am surprised how few people are wanting this feature.

[rmwphd]

related PR #1380