Rename as_promise to as_promise_object and add as_promise -> JsPromise

[hansl]

This aligns the API with other methods (e.g. as_function()) and makes it more intuitive.

BREAKING CHANGE

This is a breaking change as the JsValue::as_promise() API returns a different value. It is now more align to the rest of the API though with as_* returning the expected type.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 52.05%. Comparing base (6ddc2b4) to head (7384718).
Report is 236 commits behind head on main.

Files	Patch %	Lines
core/engine/src/value/mod.rs	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3965      +/-   ##
==========================================
+ Coverage   47.24%   52.05%   +4.81%     
==========================================
  Files         476      468       -8     
  Lines       46892    45104    -1788     
==========================================
+ Hits        22154    23481    +1327     
+ Misses      24738    21623    -3115     

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