Fix highlighting issue when printing JSON that isn't indented

[darrenburns]

Type of changes

• Bug fix
• New feature
• Documentation / docstrings
• Tests
• Other

Checklist

• I've run the latest black with default args on new code.
• I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate.
• I've added tests for new code.
• I accept that @willmcgugan may be pedantic in the code review.

Description

Previously the JSONHighlighter would do two passes with Text.highlight - the second pass would highlight JSON keys.

I've got rid of the second regex for the 2nd Text.highlight pass, and replaced it with a regex which captures the spans of strings, followed by some manual processing to determine whether they're keys or not. If the string is a key, we append the json.key Span.

Previously:

Closes #2027

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.78%. Comparing base (862416a) to head (94a1074).
Report is 1239 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2038      +/-   ##
==========================================
+ Coverage   99.54%   99.78%   +0.24%     
==========================================
  Files          73       71       -2     
  Lines        7294     7109     -185     
==========================================
- Hits         7261     7094     -167     
+ Misses         33       15      -18     

Flag	Coverage Δ
unittests	99.78% <100.00%> (+0.24%)	⬆️

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[willmcgugan]

Just a couple of requests, as discussed...

[willmcgugan]

LGTM