Add TryFromJs for TypedJsFunction and more tests

[hansl]

Includes adding TryFromJs for "()".

[codecov]

Codecov Report

Attention: Patch coverage is 71.87500% with 9 lines in your changes missing coverage. Please review.

Project coverage is 51.36%. Comparing base (6ddc2b4) to head (a66b545).
Report is 258 commits behind head on main.

Files with missing lines	Patch %	Lines
core/engine/src/object/builtins/jsfunction.rs	40.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3981      +/-   ##
==========================================
+ Coverage   47.24%   51.36%   +4.12%     
==========================================
  Files         476      470       -6     
  Lines       46892    45244    -1648     
==========================================
+ Hits        22154    23240    +1086     
+ Misses      24738    22004    -2734     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[HalidOdat]

Looks great! Just a minor nitpick :)