Initial layer-indicator support

[twadzins]

Pointer to discord thread: https://discord.com/channels/877392805654306816/1221190895806709911

This code does the following:

• Updates the dtsi erb to create behaviors and macros which are variants of the thumb, space and thumb_parang behaviors and associated macros that will, if enabled, send a unique modifier sequence when a layer key is pressed, and send F18 when the layer key is released. These behaviors are then applied to keymap.json in place of the current &thumb, &space and &thumb_parang custom mappings. The functionality is enabled with a flag USE_LAYER_INDICATORS, which is disabled by default
• Adds to the rake build: Create a BetterTouchTool preset that displays an overlay of the active keyboard layer. This preset building uses the unique key sequences sends by the above change to the dtsi

[sunaku]

Thanks for contributing this! ️❤️ I will review it when I get some time. 🧗 In the meantime, it would be helpful to get more users from the community to help test it and provide feedback. Also, I would like to expand this to support all 3 OS'es in the future.