feat(parser): add \strut alongside existing \mathstrut

[gjtorikian]

\mathstrut was already supported as a zero-width Space with a 0.7em height.

This PR adds \strut as a text-mode-sized counterpart (1.0em height), emitted as the same kind of Space event.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.8%. Comparing base (a25c6ad) to head (13b4efa).
⚠️ Report is 34 commits behind head on main.

Additional details and impacted files

Files with missing lines	Coverage Δ
src/parser.rs	99.6% <ø> (+<0.1%)	⬆️
src/parser/primitives.rs	83.7% <100.0%> (-0.2%)	⬇️

... and 5 files with indirect coverage changes

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