BIDS: Channel tagging planning

One of the missing parts of the old understanding of annotations is the idea of annotation on a single channel.

Currently in BIDS, you can mark an electrode (channel) as either good or bad as outlined in [BIDS](https://bids-specification.readthedocs.io/en/stable/04-modality-specific-files/03-electroencephalography.html#example-_electrodestsv).

Going forward, we would like to be able to flag/annotate subsets of channels for multiple different reasons with more nuance than simply "good/bad". Doing so may involve proposals to the base BIDS spec or otherwise.