Refactor ChordLyricsPair constructor to named arguments (deprecate positional args)

[martijnversluis]

Context

ChordLyricsPair has a positional-argument constructor that already carries five parameters:

constructor(
  chords = '',
  lyrics: string | null = null,
  annotation: string | null = null,
  chordObj: Chord | null = null,
  isRhythmSymbol = false,
)

The semantic-token work in the #2207 → #2229 stack adds tokenKind and tokenVariant on top of this, pushing it to seven positional args. Call sites like new ChordLyricsPair(chords, lyrics, annotation, chordObj, isRhythmSymbol, tokenKind, tokenVariant) are hard to read and error-prone (easy to pass an argument in the wrong slot, lots of null/default padding to reach the one you care about).

Proposal

Move the constructor to a single named-options object:

constructor({ chords, lyrics, annotation, chordObj, isRhythmSymbol, tokenKind, tokenVariant }: ChordLyricsPairProperties = {})

• Keep the positional signature working for now (accept either an options object or the legacy positional args) for backward compatibility.
• Mark the positional form as @deprecated and emit a deprecate(...) warning, so it can be removed in a future major.
• #set already takes an options object, so this aligns the constructor with it.

Notes

• Sweep internal call sites (parser helpers, serializer, #clone, #set) to the named form.
• Land this alongside or after the Avoid line breaks starting with rhythm symbols #2207/Normalize directives and preserve semantic chord-line tokens #2229 stack so we don't churn the same signature twice.