Per key/layer RGB underglow for 24.12

[darknao]

This adds per key / per layer RGB underglow configuration on top of 24.12.
It's mainly based on @valdur mod with additional changes to make it configurable via devicetree.

This supersedes #30.

It includes:

• Valdur's RGB mod
• Refactor, move configuration to devicetree, and some battery life optimization
• feat(split): Increase split interval during idle #682 from ZMK upstream

How to use:
More instructions are available on my config repo.
TLDR: Add and customize this snippet to the Custom Device-tree field in the layout editor:

#include <dt-bindings/zmk/rgb_colors.h>
/ {
    underglow-layer {
        compatible  = "zmk,underglow-layer";

        // example of RGB layer for the Lower keymap layer
        lower {
            bindings = <
                ______ ______ ______ ______ ______                                                          ______ ______ ______ ______ ______
                PURPLE   PINK   PINK   PINK   PINK   PINK                                            ______   GOLD ORANGE ORANGE ORANGE    RED
                PURPLE ______ ______ ORANGE ______ ______                                            ______ YELLOW YELLOW YELLOW ORANGE ______
                PURPLE ______ ORANGE    RED ORANGE ______                                            ______ YELLOW YELLOW YELLOW ORANGE ______
                  BLUE ______    RED    RED    RED ______ ______ ______ ______  ______ ______ ______ ______ YELLOW YELLOW YELLOW    RED   BLUE
                ______ ______ ORANGE ORANGE ORANGE        ______  GREEN ______  ______ ______ ______        YELLOW YELLOW YELLOW    RED ______
                >;
            layer-id = <LAYER_Lower>;
        };

        // This defines another RGB layer
        another-layer {
            bindings = < [...] >;
            layer-id = <LAYER_Name>;
        };
    };
};

______ means the RGB is off. You can use predefined color name or RGB hex code like 0xFF0000 for red.
Full example on the Layout Editor here

To preserve battery life, I recommend enabling the ZMK_RGB_UNDERGLOW_AUTO_OFF_IDLE KConfig to automatically turn off underglow after ZMK_IDLE_TIMEOUT ms (default to 30 seconds).
From my limited experience, you can expect around one week of battery life with no RGB on the base layer.