* Use TAPPING_TERM constant (and redefine value to 200ms)
* change TAPPING_TOGGLE to 2 to require two taps to lock in layer 4
* add support for Shift-Command 3/4/5 key combinations that are used in
macOS Catalina
* avoid false positive tap detecion for RMOD3 when the whole sequence of
pressing RMOD3, tapping another key and releasing RMOD3 took less than
TAPPING_TERM milliseconds.
* replace SEND_STRING with tap_code()/tap_code16(), saving ~860 bytes in
compiled firmware size.
