feat: Support multi-letter identifiers in font groups

[gjtorikian]

This PR closes #45 by following the proposal in #45 (comment).

When the argument to a font-modifier command (\mathrm, \mathit, \mathnormal, etc.) is a run of two-or-more ASCII alphabetic characters, this PR now emits a single Content::Identifier event so the renderer can produce one <mi> for the whole name. Non-alphabetic arguments still fall back to the per-character path.

(In thinking about the breaking change noted in #51 (comment), I went around looking at issues where other breaking changes could be implemented, all at once.)

[codecov]

Codecov Report

❌ Patch coverage is 96.47059% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.3%. Comparing base (e2b4f8d) to head (53b7b57).
⚠️ Report is 19 commits behind head on main.

Files with missing lines	Patch %	Lines
src/parser/primitives.rs	86.6%	2 Missing ⚠️
src/mathml.rs	94.4%	1 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
src/event.rs	74.1% <ø> (ø)
src/parser.rs	99.6% <100.0%> (+<0.1%)	⬆️
src/mathml.rs	82.6% <94.4%> (+0.8%)	⬆️
src/parser/primitives.rs	84.1% <86.6%> (+0.3%)	⬆️

... and 1 file with indirect coverage changes

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[carloskiki]

I'm still torn on this, I'll think about it more before we merge.