feat(parser): accept braced dimension arguments and \Space support

[gjtorikian]

This PR is purely for MathJax / KaTeX compatability.

Plain TeX requires a bare form (e.g. \hskip 1em), but KaTeX and MathJax also tolerate \hskip{1em}. This PR adds lex::dimension_or_braced and lex::glue_or_braced wrappers that transparently unwrap a single {...} group around the dimension, and use them for \kern, \hskip, \mkern, \mskip.

It also adds the MathJax \Space{width}{height}{depth} MathJax extension. To support it, Event::Space gains a third depth: Option<Dimension> field, and the MathML writer now emits it as the depth attribute on <mspace> when present.

[codecov]

Codecov Report

❌ Patch coverage is 92.94118% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.5%. Comparing base (d385cae) to head (0049651).

Files with missing lines	Patch %	Lines
src/parser/primitives.rs	83.3%	6 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
src/event.rs	75.5% <ø> (+1.4%)	⬆️
src/mathml.rs	81.1% <100.0%> (+<0.1%)	⬆️
src/parser/lex.rs	94.4% <100.0%> (+1.0%)	⬆️
src/parser/primitives.rs	85.4% <83.3%> (+1.6%)	⬆️

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