Change MIDI velocity implementation to allow direct control of velocity value (#9940)

* Initial changes to expose "real" MIDI velocity

* Change MI_VELU and MI_VELD to fit new logic

* Apply cleanups from fauxpark's review

my bad on the errant whitespace and else {} placements

* Slight changes to MI_VELD to match values when velocity is falling to when it is rising

* Add keycode MI_VEL_0

* Update compute_velocity() to handle MI_VEL_0

* Store velocity as 7 bits to hard cap value at 127

quantum/process_keycode/process_midi.h

@@ -35,7 +35,7 @@ typedef union {
     struct {
         uint8_t octave : 4;
         int8_t  transpose : 4;
-        uint8_t velocity : 4;
+        uint8_t velocity : 7;
         uint8_t channel : 4;
         uint8_t modulation_interval : 4;
     };