handwired/gamenum Refactor, Configurator support and readme cleanup (#4563)

handwired/gamenum Refactor, Configurator support and readme cleanup (#4563)

* handwired/gamenum: refactor

- layout macro KEYMAP renamed to LAYOUT
  - white space changes for alignment
- default keymap
  - now uses #include QMK_KEYBOARD_H
  - updated layout macro names
  - white space changes (for readability)

* handwired/gamenum: Configurator support

* handwired/gamenum: readme cleanup

- renamed file to lowercase
- updated to match current QMK template more closely
- edits to reflect the other changes in this PR