Add `gabc` code block

[rysson]

Is your feature request related to a problem? Please describe.

HedgeDoc already support abc format. It will be nice to use gabc format too (gregorian abc) to type neumes. GABC notation is described here: https://gregorio-project.github.io/gabc/index.html I've found JS library: https://github.com/bbloomf/jgabc

Describe the solution you'd like

Add gabc as gabc code block.

```gabc
(c4) SAl(c)ve,(e) Re(g')gí(h)na, *(g.) ma(h)ter(j') mi(i)se(h)ri(g)cór(h')di(g)æ :(g.) (;)
Vi(j)ta,(g') dul(h)cé(ff)do,(d.) (;)
et(e) spes(f) no(g')stra,(e) sal(ed)ve.(c.) (::)
```

Visible as interactive neumes like in https://bbloomf.github.io/jgabc/transcriber.html or just image at least

Describe alternatives you've considered

Only other sites and tools (like lualatex with greogorio).

Additional context

No response