fix: use parity-aware regex to handle escaped backslashes in JSON str…

[Shuklax]

What kind of change does this PR introduce?
Bugfix / Refactoring.

Issue Number:
Closes #2013

Summary:
This PR provides a robust, performance-optimized fix for the JSON string parsing issue where trailing escaped backslashes (e.g., "\\") break the editor's syntax highlighting.

Technical Advantage over other proposals:
Unlike solutions using manual character-counting loops, this PR implements a parity-aware regex lookbehind: (?<=(^|[^])(?:\)*)".

Efficiency:
It leverages the native V8/C++ regex engine, which is more performant than manual JavaScript loops for large payloads.

Maintainability:
It solves the problem at the regex definition level, keeping the core parsing logic clean and declarative.

Improved Coverage:
It also updates regexString to ensure that full-string matches are correctly identified when they contain escaped backslashes.

[github-actions]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (58506bc) to head (6306a03).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #2030   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           30        30           
  Lines          633       633           
  Branches       196       196           
=========================================
  Hits           633       633           

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