Propose new delimiter to split NABC sections

[lbssousa]

Following https://github.com/gregorio-project/gregorio/issues/1662#issuecomment-2854678153, I would like to propose a change in NABC syntax, introducing a new delimiter to split multiple NABC sections, and reserving current delimiter | for alternating between GABC and NABC sections.

Current syntax example:

Text(gabc1|nabc11|nabc12|gabc2|nabc21|nabc22)

Proposed syntax example (using & as hypothetical delimiter for NABC sections):

Text(gabc1|nabc11&nabc12|gabc2|nabc21&nabc22)

This change would make it easier to implement syntax highlighting for GABC+NABC code.