Mark JsNativeError's constructors const

[CrazyboyQCD]

Use constant to reduce binary size.
https://godbolt.org/z/YEq4hW49n

[codecov]

Codecov Report

Attention: Patch coverage is 61.90476% with 8 lines in your changes missing coverage. Please review.

Project coverage is 52.85%. Comparing base (6ddc2b4) to head (332770f).
Report is 273 commits behind head on main.

Files with missing lines	Patch %	Lines
core/engine/src/error.rs	61.90%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4020      +/-   ##
==========================================
+ Coverage   47.24%   52.85%   +5.61%     
==========================================
  Files         476      482       +6     
  Lines       46892    46723     -169     
==========================================
+ Hits        22154    24696    +2542     
+ Misses      24738    22027    -2711     

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[jedel1043]

Nice improvement!

[jedel1043]

Is this guaranteed though? IIRC, Box<T> has the same layout as &T only for T: Sized, but for unsized types they could always change the pointer metadata layout.

If we don't want to rely on this, we can just use Option<Box<str>>, which should be const-compatible.