[exec - function] Add constructor property to function.prototype

[croraf]

Intended to solve this #533 .
It is actually a couple of lines of change, but I based it on refactor PR which should be merged first: #529

I basically added proto.set_field("constructor", val.clone()); to function.prototype. val here is:

let new_func = Object::function(func);
let val = Value::from(new_func);

https://github.com/boa-dev/boa/pull/534/files#diff-d7bd296d9ca534161e198b8c141ca51fR140-L149

Someone has to doublecheck if setting the val.clone() here is correct.

[codecov]

Codecov Report

Merging #534 into master will increase coverage by 0.00%.
The diff coverage is 42.85%.

@@           Coverage Diff           @@
##           master     #534   +/-   ##
=======================================
  Coverage   68.33%   68.34%           
=======================================
  Files         172      172           
  Lines       10495    10496    +1     
=======================================
+ Hits         7172     7173    +1     
  Misses       3323     3323           

Impacted Files	Coverage Δ
boa/src/exec/mod.rs	68.64% <42.85%> (+0.10%)	⬆️

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[RageKnify]

@croraf I think #642 fixed this #533, this PR can be closed, right?

[HalidOdat]

@croraf I think #642 fixed this #533, this PR can be closed, right?

Yep #642 did add that, so closing this.