add: LED widgets

[crides]

Fixes #1003

Currently has the config in C, see example here. You also need to setup a LED device as shown here. It would be better to put the config in devicetree, however there are some macro issues to be resolved before that could be done. Having the config in devicetree means that the config is better structured, and is less janky (see below) compared to the current C implementation. However, it also means that rebuilding is slower due to the nature of processing devicetrees in Zephyr.

Issues:

• The C config is janky, and uses embedded arrays to allow for easier config and macro generation. There are magic numbers for the (max) size of those embedded arrays. This could either be solved by having actual configuration items for them, or have smarter macros (probably hard)
• Currently this only supports single LEDs, whether PWM or not should work fine; support for addressable LED isn't a technical problem, and I'll see what needs to be done to adapt the config/interface
• No animations supported

[alinelena]

i have played with the layer led widget and works for "normal" layers but if you use a conditional layer the indicator does not work when this layer is activated.

example here
https://github.com/alinelena/zmk/tree/mlego2/app/boards/shields/mlego2x2