Fix edge case in JsonTemplateLayout with certain escaped sequences

[jtduffy]

Resolves #2184

• Use compiled regex pattern to detect un-escaped ", sequences
• New unit tests

Regarding the use of regex, I ran several tests iterating over the getIndexToModifyJson method 1 million times, and the total time was around 450ms for each test.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.67%. Comparing base (9124406) to head (51a81f1).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #2187   +/-   ##
=========================================
  Coverage     70.67%   70.67%           
+ Complexity     9996     9995    -1     
=========================================
  Files           842      842           
  Lines         40363    40363           
  Branches       6116     6116           
=========================================
+ Hits          28525    28527    +2     
+ Misses         9084     9081    -3     
- Partials       2754     2755    +1     

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