Add way to create JsString from ASCII literal without heap allocation

[CrazyboyQCD]

Use js_string macro to create JsString from an ASCII literal usually causes heap allocation.
This adds a way to create JsString from ASCII literal without heap allocation.

[codecov]

Codecov Report

Attention: Patch coverage is 89.65517% with 3 lines in your changes missing coverage. Please review.

Project coverage is 51.04%. Comparing base (6ddc2b4) to head (138a6d9).
Report is 211 commits behind head on main.

Files	Patch %	Lines
core/string/src/lib.rs	89.65%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3922      +/-   ##
==========================================
+ Coverage   47.24%   51.04%   +3.79%     
==========================================
  Files         476      467       -9     
  Lines       46892    44492    -2400     
==========================================
+ Hits        22154    22709     +555     
+ Misses      24738    21783    -2955     

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