Add step sequencer feature (#9703)

* sequencer: create togglable feature

* sequencer: add support for steps

* sequencer: add support for tempo and resolutions

* sequencer: schedule a message print at the right frequency

* sequencer: send a hardcoded note

* sequencer: add support for 8 tracks

* sequencer: play several notes simultaneously

* sequencer: only play the active tracks for a given step

* sequencer: change the default behavior of the track toggler

* sequencer: make number of tracks and track notes customizable

* sequencer: move the keycodes down

Not adding them at the end of the list apparently risks breaking
compatibility with VIA.

Source: https://github.com/qmk/qmk_firmware/pull/9703#discussion_r459202733

* sequencer: add unit tests

* sequencer: add unit test for matrix_scan_sequencer

* sequencer: expose internal state for better unit-testability

* sequencer: add unit tests for matrix_scan_sequencer

quantum/quantum.c

@@ -229,6 +229,9 @@ bool process_record_quantum(keyrecord_t *record) {
             process_record_via(keycode, record) &&
 #endif
             process_record_kb(keycode, record) &&
+#if defined(SEQUENCER_ENABLE)
+            process_sequencer(keycode, record) &&
+#endif
 #if defined(MIDI_ENABLE) && defined(MIDI_ADVANCED)
             process_midi(keycode, record) &&
 #endif
@@ -633,6 +636,10 @@ void matrix_scan_quantum() {
     matrix_scan_music();
 #endif
 
+#ifdef SEQUENCER_ENABLE
+    matrix_scan_sequencer();
+#endif
+
 #ifdef TAP_DANCE_ENABLE
     matrix_scan_tap_dance();
 #endif