New syntax for separating gabc and nabc voices

[davidweichiang]

Closes #1667.

• & advances to the next voice, but doesn't cycle back to the first voice.
• | is for cycling back to the first voice, but for backwards compatibility also can be used in place of &.

Currently, nabc-lines: is still required.

[davidweichiang]

This works as far as I know, but I just left it as a draft in case there was discussion about what the syntax should look like.

[rpspringuel]

What would removing the nabc-lines header requirement involve?

[davidweichiang]

My recollection is that the nabc voices are stored in an array, so the nabc-lines: is needed in order to malloc the array. Removing the nabc-lines: header would require adding code to realloc the array whenever a new voice is seen.

Tangentially, this is also related to a question that came up on the mailing list (https://groups.google.com/g/gregorio-users/c/IwslfmnTPDk); currently \gabcsnippet does not allow nabc because it doesn't offer a way to pass the nabc-lines: header.

[rpspringuel]

So which would be easier, in your opinion, removing the nabc-lines header or creating a way to pass headers to \gabcsnippet?

[davidweichiang]

I think they're both doable but the latter would be easier (and orthogonal to this PR).

I actually kind of like the nabc-lines header, since it's a global property of a score.