[Keymap] Added Xerpocalypse's layout (#6732)

[Keymap] Added Xerpocalypse's layout (#6732)

* Added Xerpocalypse's layout

+ Number row and symbols are switched compared to default TMO50 layout
+ Right-hand spacebar acts as backspace and a hold-layer for layer 2.

* Update keyboards/tmo50/keymaps/xerpocalypse/keymap.c

Removed unnecessary #define

Co-Authored-By: noroadsleft <18669334+noroadsleft@users.noreply.github.com>
* Update keyboards/tmo50/keymaps/xerpocalypse/keymap.c

Changed keymap to use KC_UNDS instead of custom-defined keycode

Co-Authored-By: noroadsleft <18669334+noroadsleft@users.noreply.github.com>