GROOVY-12272: Validate JSON string escapes where they are read

[paulk-asert]

JsonLexer's string branch appended one character at a time and, at every unescaped quote, re-validated the whole accumulated token by copying it to a String and running two regular expressions over it. For a well formed string the closing quote is the first unescaped one, so that happened once. For a string holding an invalid escape the validation could never succeed, so the loop consumed the rest of the document and paid the cost again at every quote it passed, giving O(n^2) behaviour on input an author controls.

Measured on a document of the shape {"k":"\q" followed by n quotes, parsed with JsonSlurperClassic:

quotes before after
2,000 42 ms 4 ms
4,000 67 ms 0 ms
8,000 220 ms 0 ms
16,000 874 ms 0 ms

Read and check each escape sequence where the backslash is found instead. The scan becomes linear, the accepted language is unchanged, and a bad escape is now reported at its own position rather than after the document has been consumed to its end. Escape-state tracking is no longer needed, since consuming the sequence in place is what distinguishes an escaped quote from a closing one.

Note the reach is wider than the parser: JsonOutput.prettyPrint(String) lexes through the same class, so it shared the behaviour.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 77.77778% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.1755%. Comparing base (5f56279) to head (db9e70b).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
...oovy-json/src/main/java/groovy/json/JsonLexer.java	77.7778%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@                Coverage Diff                 @@
##               master      #2809        +/-   ##
==================================================
+ Coverage     70.1516%   70.1755%   +0.0239%     
- Complexity      35828      35845        +17     
==================================================
  Files            1562       1562                
  Lines          132523     132542        +19     
  Branches        24379      24384         +5     
==================================================
+ Hits            92967      93012        +45     
+ Misses          31140      31114        -26     
  Partials         8416       8416                

Files with missing lines	Coverage Δ
...oovy-json/src/main/java/groovy/json/JsonLexer.java	80.5556% <77.7778%> (-2.4232%)	⬇️

... and 9 files with indirect coverage changes

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[testlens-app]

✅ All tests passed ✅

🏷️ Commit: db9e70b
▶️ Tests: 110470 executed
⚪️ Checks: 31/31 completed

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