Make well_known_symbols functions pub

[tj825]

This Pull Request makes the well_known_symbols functions -- boa_engine::symbol::JsSymbol::iterator() -> JsSymbol etc -- pub instead of pub(crate).

This is supposed to allow embedders to get hold of well-known symbols in Rust code, e.g. to implement iterable native objects, etc.

(Have I missed some better way to do this?)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (3a4fe6f) 44.62% compared to head (6b2d32f) 44.62%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3465   +/-   ##
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  Coverage   44.62%   44.62%           
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  Files         487      487           
  Lines       50541    50541           
=======================================
  Hits        22554    22554           
  Misses      27987    27987           

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[jedel1043]

Yeah, I would be very surprised if the spec removed any well known symbols in the future, so I think it is okay to expose them as public.

[nekevss]

Makes sense to me. 😄 Thanks for the contribution!