handwired/retro_refit: refactor, Configurator support and readme update (#4899)

handwired/retro_refit: refactor, Configurator support and readme update (#4899)

* handwired/retro_refit: refactor

- retro_refit.h
  - updated to use #pragma once include guard
  - refactored layout macro to not auto-prepend KC_ to keycodes
  - renamed to LAYOUT (from KEYMAP)
  - aligned for readability
- default keymap.c
  - now uses #include QMK_KEYBOARD_H
  - removed unused fn_actions and action_get_macro blocks

* handwired/retro_refit: Configurator support

* handwired/retro_refit: readme update

Updated readme to format of current QMK template.