* Improve the name of first parameter to `rgb_assign_color()` to reflect it's a bitmask of which LEDs to assign the color to.
* Formatting changes to more easily compare similar code.

* Each `T_CONFIG_...` option is listed on its own line.  This improves the ability to diff changes.  It also allows reading the options in code editor. (no ultra-long lines!)

* Align the elmeents of of `sub_prompts[]`.  This allows easier understanding of the differences between each array element.

* Use named initializers for `main_cfg` and `main_prompt` variables.  This simply adds clarity when reviewing the code.

No behavioral change.  Just makes it harder to make mistakes later.  Static assertions added for the same reason.

Rewrote function for setting brightness from menu for increased clarity.

Fix LED config name to reflect its purpose:
Specifically, it doesn't store the LED brightness, but rather stores a divisor value (i.e., the inverse of storing the brightness).

Store the LED effect as an enum.
(self-documenting, better debugger visualizations, ...)

Delete some dead code.

Eventually, will have this use the user's chosen color.
Importantly, it uses higher brightness for edge pixels than for those facing same direction as the screen.