Fix #581: Overzealous Mark Occurrences for single characters

[bobbylight]

Discussed as part of the issue in #575. The "Mark Occurrences" support usually highlights all instances of any token of type "identifier." Unfortunately many TokenMaker implementations style the characters ,, ;, and . (and possibly others) as identifiers for aesthetic purposes only. This has the side effect of the Mark Occurrences support highlighting all instances of them if the caret pauses near them.

At bets, this is visually surprising. At worst, it can cause the editor to hang for very large files with many instances of the character at the caret.

We should skip marking occurrences of non-word characters at any location. In 4.0.0 we can look at a more robust solution.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 80.35%. Comparing base (294a830) to head (ef03839).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...fe/ui/rsyntaxtextarea/DefaultOccurrenceMarker.java	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #582      +/-   ##
============================================
- Coverage     80.36%   80.35%   -0.01%     
- Complexity     7778     7781       +3     
============================================
  Files           178      178              
  Lines         30232    30235       +3     
  Branches       3909     3910       +1     
============================================
  Hits          24295    24295              
- Misses         4262     4267       +5     
+ Partials       1675     1673       -2     

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