feat(graalvm): add support for navigator.userAgent

[sgammon]

Summary

Adds navigator.userAgent to the JS context.

• JS API: Navigator #1275

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 48.24%. Comparing base (9d9be0e) to head (4bac742).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...otlin/elide/runtime/javascript/NavigatorBuiltin.kt	64.28%	4 Missing and 1 partial ⚠️
...vm/src/main/kotlin/elide/runtime/gvm/js/JsError.kt	0.00%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1277   +/-   ##
=======================================
  Coverage   48.24%   48.24%           
=======================================
  Files         499      500    +1     
  Lines       18277    18295   +18     
  Branches     3234     3236    +2     
=======================================
+ Hits         8817     8826    +9     
- Misses       8433     8441    +8     
- Partials     1027     1028    +1     

Flag	Coverage Δ
jvm	48.24% <50.00%> (+<0.01%)	⬆️
lib	48.24% <50.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
...vm/src/main/kotlin/elide/runtime/gvm/js/JsError.kt	30.00% <0.00%> (-2.61%)	⬇️
...otlin/elide/runtime/javascript/NavigatorBuiltin.kt	64.28% <64.28%> (ø)

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