Update personal userspace and keymaps + improve custom functionality (#5289)

Update personal userspace and keymaps + improve custom functionality (#5289)

* Change how desktop commands work

* Add DST_MOD_MASK as a config option

* DST_RMV → DST_REM

* Add melody96:konstantin keymap

* Update custom SEND_STRING

* Move feature flags from userspace into keyboard rules

* Use "Vo-", "Vo+" instead of "VoD", "VoU" in keymap comments

* Add RGB controls and numpad Unicode to Melody96 keymap

* Add RGB_SET keycode to Melody96 and RGB files to userspace

* Generate UNICODE and UNICODEMAP constants using macros

* Avoid collisions with X_* send string constants

* Use two spaces before inline comments

* Add _keymap versions of other custom Quantum functions

Not added: eeconfig_update_keymap, eeconfig_read_keymap

* Switch to UNICODEMAP in keyboard rules

* Make toggle_numpad a nested function in process_record_user

* Set Melody96 underglow color to Godspeed blue on EEPROM reset

* Remove most _keymap and _user definitions in userspace

Some keyboards misuse _user functions by defining them in the base files
instead of the corresponding _kb functions (especially led_set_user and
matrix_init_user). Until this is fixed (#5148), I've removed definitions
in my userspace that could cause linking collisions.

* Update GODSPEED_BLUE values and RGB mode keys

* Add GODSPEED_YELLOW color

* Set preferred intervals for rgblight effects

* Update tap dance function names

* Replace td_lshift_fn with generic td_mod_layer, add TD_RCTL_FN

Move TD_FN_RCTL after TD_RCTL_FN

* Replace td_fn_rctrl with generic td_layer_mod

* Add blank lines, prefer explicit initialization

* ACTION_TAP_DANCE_DOUBLE_MODS → ACTION_TAP_DANCE_DOUBLE_MOD

* Update Godspeed colors

* Add media controls to Melody96 keymap

* Add SysRq, Break combos and other keys to Melody96 keymap