Added bending to MIDI (#2740)

This is done via MI_BENDD and MI_BENDU. At the moment the value is
hardcoded and cannot be adjusted (future commit?) and is the max for the
`midi_send_pitchbend` function (up or down).

`MI_BENDD` and `MI_BENDU` both require `#define MIDI_ADVANCED`

MIDI pitch bend was already implemented in `protocol/midi.c`, I merely
added the keycodes to trigger them. :) (thanks to Jack, two years ago
in commit fb4fe52c apparently)

quantum/quantum_keycodes.h

@@ -383,6 +383,9 @@ enum quantum_keycodes {
     MI_MOD, // modulation
     MI_MODSD, // decrease modulation speed
     MI_MODSU, // increase modulation speed
+
+    MI_BENDD, // Bend down
+    MI_BENDU, // Bend up
 #endif // MIDI_ADVANCED
 
     // Backlight functionality