LibJS: Add a [[Realm]] getter to FunctionObject and use it where needed

[IdanHo]

Defined by https://tc39.es/ecma262/#sec-ordinaryfunctioncreate step #17 and by https://tc39.es/ecma262/#sec-createbuiltinfunction step #6.

(Also includes a small fix to String's constructor to make it use this underlying behaviour)

Fixes 30 test262 test cases.

[davidot]

I used global_object in array_species_create (not directly) perhaps this can also use the realm() for clarity?

[IdanHo]

I used global_object in array_species_create (not directly) perhaps this can also use the realm() for clarity?

The specification actually requires that ArraySpeciesCreate will use the GetFunctionRealm abstract operation, so i added a commit to fix that.