Feat: Add support for Unicode boundaries

[mkslanc]

Issue #, if available: #5232

Description of changes:
Add support for Unicode boundaries and detection methods

• Introduced Unicode boundaries support in text search
• Added supportsLookbehind and supportsUnicodeFlag methods in lang for feature detection
• Implemented fallback to ASCII boundaries when the browser does not support look-behinds
• Implemented fallback to old behaviour (without unicode support) in rare edge cases for backward compatibility

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[codecov]

Codecov Report

Patch coverage: 95.55% and project coverage change: +0.01% 🎉

Comparison is base (31bbd6a) 87.23% compared to head (a775fcb) 87.24%.
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5265      +/-   ##
==========================================
+ Coverage   87.23%   87.24%   +0.01%     
==========================================
  Files         565      565              
  Lines       45248    45330      +82     
  Branches     6920     6934      +14     
==========================================
+ Hits        39470    39548      +78     
- Misses       5778     5782       +4     

Flag	Coverage Δ
unittests	87.24% <95.55%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
src/lib/lang.js	73.33% <80.00%> (+0.60%)	⬆️
src/search.js	91.73% <100.00%> (+0.61%)	⬆️
src/search_test.js	99.63% <100.00%> (+0.02%)	⬆️

... and 3 files with indirect coverage changes

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