VoicingDictionary should enable mapping multiple chord symbols to one set of intervals

[felixroos]

Copied from #224 (comment)

This should be done before publish to avoid a breaking change (if we decide to do it)

Before:

export const lefthand: VoicingDictionary = {
  m7: ["3m 5P 7m 9M", "7m 9M 10m 12P"],
  /* more symbols */
}

after:

const lefthand = [
  [['m7','m9'], ["3m 5P 7m 9M", "7m 9M 10m 12P"]],
  /* more symbols */
];

This format allows mapping more than one symbol to a set of intervals. This resembles how many chord symbols are sometimes used synonymously. For example, in a jazz leadsheet, a "m7" is mostly played with a 9 (like above). If the sheet contains a "m9" instead, it is played exactly the same.

Additionally, this makes it possible to define the same symbol multiple times, which could be useful for some scenarios. VoicingDictionary.lookup should therefore filter all dictionary items by the given symbol and concat all sets that match.

[felixroos]

defining shell voicings would be much shorter with this:

const shell = [
  [["m7", "m9", "m11", "m13", /* many many more*/], ["3m 7m", "7m 10m"]],
  /* more symbols */
];

[danigb]

I think we should not export the data directly. See #230