Add splitByElement() method for parsing nested list or set

[yunlingTao]

Added splitByElement() method in gremlin-test which splits strings without breaking elements containing commas inside brackets or braces.
It replaces split(",") in list and set parsing to handle structures like s[l[1,2,3], l[2,3,4]].

[codecov-commenter]

Codecov Report

❌ Patch coverage is 80.95238% with 4 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (3.8-dev@470ba52). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...che/tinkerpop/gremlin/features/StepDefinition.java	80.95%	0 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             3.8-dev    #3216   +/-   ##
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  Coverage           ?   76.77%           
  Complexity         ?    14942           
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  Files              ?     1159           
  Lines              ?    72502           
  Branches           ?     8041           
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  Hits               ?    55667           
  Misses             ?    13816           
  Partials           ?     3019           

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[andreachild]

Would be good to add a scenario to a feature test would would execute this new logic. Perhaps adding a scenario to the Fold.feature which does something like g.V().fold().fold() which will result in a list of lists?

[andreachild]

Do other GLVs need a similar change? For example the parsing logic for lists and sets in feature-steps.js and feature_steps.py.