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.h

@@ -68,6 +68,11 @@ extern layer_state_t default_layer_state;
 extern layer_state_t layer_state;
 #endif
 
+#if defined(SEQUENCER_ENABLE)
+#    include "sequencer.h"
+#    include "process_sequencer.h"
+#endif
+
 #if defined(MIDI_ENABLE) && defined(MIDI_ADVANCED)
 #    include "process_midi.h"
 #endif