TINKERPOP-3260: Browser testing for gremlin-javascript

[Iryna-Kahamlyk]

JIRA: https://issues.apache.org/jira/browse/TINKERPOP-3260

Summary

Adds two-tier smoke testing:

• Tier 1: Bundles the package and verifies that it loads successfully in headless Chromium, catching Node-only imports at build time.
• Tier 2: Verifies that browser transport can successfully round-trip HTTP requests to a real Gremlin Server.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.71%. Comparing base (a28cd1f) to head (590dc7d).
⚠️ Report is 732 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3674      +/-   ##
============================================
+ Coverage     76.35%   76.71%   +0.35%     
- Complexity    13424    14345     +921     
============================================
  Files          1012     1037      +25     
  Lines         60341    64819    +4478     
  Branches       7075     7704     +629     
============================================
+ Hits          46076    49728    +3652     
- Misses        11548    11995     +447     
- Partials       2717     3096     +379     

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[kenhuuu]

We shouldn't do this as this is actually the best way to test Gremlin Server's CORS support.