Add SMC key for charging MagSafe LED

[nicolasmaia]

For context, I was hoping to have the MagSafe LED of my M2 Macbook Air change colors when the battery charging hits 80%, and opened an issue with the GNOME Extension I use (Battery-Health-Charging), and the maintainer recommended opening an issue here, so that Asahi could expose this information for GNOME shell extensions to use.

For reference, this can be done on MacOS using the battery utility. Cf: https://stackoverflow.com/questions/75738532/control-magsafe-leds-on-apple-silicon-macbook/77815493#77815493

Original issue that added this feature to the battery program:
actuallymentor/battery#71