Add a js_error! macro to create opaque errors

[hansl]

And as a bonus since I needed it, simplify comparison of strings and add PartialEq<&str> for JsString.

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.

Project coverage is 51.52%. Comparing base (6ddc2b4) to head (e74ca1c).
Report is 211 commits behind head on main.

Files	Patch %	Lines
core/string/src/lib.rs	33.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3920      +/-   ##
==========================================
+ Coverage   47.24%   51.52%   +4.28%     
==========================================
  Files         476      467       -9     
  Lines       46892    44803    -2089     
==========================================
+ Hits        22154    23085     +931     
+ Misses      24738    21718    -3020     

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[jedel1043]

Looks perfect! Thank you!

[nekevss]

Looks good to me :)