✨ feat(mq-lang): add squeeze_whitespace

[harehare]

Summary

Joining or re-flowing text from multiple sources commonly leaves runs of extra spaces/tabs/newlines behind, with no single builtin to clean that up. Adds squeeze_whitespace(s), collapsing whitespace runs to one space and trimming the ends. String-only (mirrors normalize_newlines' own convention): gsub/trim on a markdown node only behave correctly for leaf nodes (text, code, ...) — on a container node (heading, paragraph, ...) they flatten it to its rendered source text instead, which isn't something this function should paper over.

Type of Change

• 🐛 Bug fix
• ✨ New feature
• ♻️ Refactor
• 📝 Documentation
• ⚡ Performance
• ✅ Test
• 📦 Build / dependencies
• 👷 CI

Checklist

• I ran cargo fmt and cargo clippy and addressed any warnings
• I ran just test-all and all tests pass
• I added or updated tests covering this change
• I updated relevant documentation (/docs, crate README.md) if needed
• I added a changelog entry if this is a user-facing change

Additional Context

[codspeed]

Merging this PR will not alter performance

✅ 27 untouched benchmarks
⏩ 35 skipped benchmarks¹

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_{Comparing feat/squeeze-whitespace (ff0fde8) with feat/replace-prose (6514893)}

Footnotes

1. 35 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩