This implements a new `FocusSerial` command: `plugins`. The `plugins` command
will reply back with a list of plugins enabled in the firmware. The list is not
exhaustive, only plugins that opt-in to this mechanism will be listed. It is
opt-in, because for a lot of plugins, having them listed isn't useful in a
practical sense.
The goal with this feature is to allow Chrysalis to detect plugins that would
affect what keys it offers, or which additional settings it displays, and do so
in a consistent way. This is why IdleLEDs has an `onNameQuery` handler too, even
though it can be detected otherwise: for consistency.
Signed-off-by: Gergely Nagy <algernon@keyboard.io>