Feature request: add center/right alignment options for above-lines text

[lbssousa]

Motivation

Above-lines text (<alt>...</alt> and the note-level [alt:text] field) is
always left-aligned to its anchor note/glyph: the text's left edge lines up
with the anchor's left edge, and the text extends rightward from there.
There's no way to center it over the anchor or right-align it against it.

This makes it awkward to annotate anything that should read as ending at a
note rather than starting there (editorial marks placed at the close of a
melisma or phrase, footnote-style callouts anchored to a cadence,
rubrics anchored to a note near the ending of the line, or simply
centering a short label over a single note instead of having it drift to the
right).

Proposed syntax

Add optional per-occurrence horizontal alignment to the existing syntax,
plus a document-wide default:

• Extend syllable-level tag <alt> with alignment modifiers for that occurrence:
  <alt:l> (left), <alt:c> (center), and <alt:r> (right)
• Extend note-level field: [alt:text;l] (left), [alt:text;c] (center),
  and [alt:text;r] (right)
• New macro \gresetabovelinestextalignment{left|center|right} to set the
  document-wide default for <alt>...</alt> and [alt:text]
  (defaults to left); per-occurrence markers override the default.

Example:

A<alt:c>Alleluia</alt>(fg) B[alt:cadence;r](g!hwi) (::)

Backward compatibility

<alt> keeps working exactly as it does today: <alt:l>, <alt:c>, and
<alt:r> are new tag names that were not valid gabc before, so no existing
score using <alt> changes its rendered output.

The note-level [alt:text] field is almost purely additive: the
document-wide default (\gresetabovelinestextalignment) defaults to left,
matching the old behavior, and the new ;l/;c/;r suffix only takes
effect when it appears literally, unescaped, at the very end of text. The
one edge case is a score where text already ended in a literal, unescaped
;l, ;c, or ;r: that suffix is now stripped and treated as an
alignment marker instead of literal text, changing the rendered output. See
UPGRADE.md for the workaround (escaping the semicolon with $).