feat: wordBoundary anchor

[mdjastrzebski]

Summary

Implement wordBoundary anchors (as well as their negation).

Checklist

• Implementation
• Tests
• API docs
• README docs (if relevant)
• Example docs & tests (if relevant)

Test plan

Add relevant tests.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.40%. Comparing base (aa585b1) to head (418fe1f).

Additional details and impacted files

@@ Coverage Diff @@ ## main #62 +/- ## ======================================= Coverage 99.39% 99.40% ======================================= Files 14 14 Lines 165 167 +2 Branches 40 40 ======================================= + Hits 164 166 +2  Misses 1 1 

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