Add light utility functions

[xnmet]

My use case is:

void cyan(led_t *currentKeyLedColors) {
  setAllKeysToBlank(currentKeyLedColors);
  setLetterKeysColor(currentKeyLedColors, naiveDimRGB(0x0195AF));
}

I am not sure about the need of adding an example as miamiNights uses setModKeys but is disabled.

Note

For completeness it could have:

• [1] numberKeys
• [2] numberKeys with - =
• [3] letterKeys with ,./;'[]\

But for what I see, people use more 2 and 3 (without \) and a setModKeys variation with \.