[Keymap] bingocaller's DZ60 MacOS keymap (#5914)

[Keymap] bingocaller's DZ60 MacOS keymap (#5914)

* Added customisations and README

* Tweak keymap: word traversal/deletion

* Add w and b word traversal/deletion keycodes.
* Add fine volume control key codes, but don't use them, because they
  conflict with other key codes. `A` somehow got remapped to fine
  volume up.

* Set mousekey delay to zero

* Use SAFE_RANGE for key codes.

* Update keymap and README

Add new mouse-specific layer 3, activated by pressing and holding space.
Add brightness controls to layer 4 (previously, layer 3).
Update README:
  * New keyboard-layout mockup image.
  * Add actual link to kbdfans.cn.
  * Update layer descriptions.

* Fix indentation in keymap.c

* Use _______ over KC_TRNS to increase readability

* Custom keys: use #define over process_record_user

* Use enum for naming layers

* Rename README.md -> readme.md