Remove @:overload from Reflect.makeVarArgs

[Simn]

While working on #11843 I came across this function which has an @:overload in order to support both Array<Dynamic>->Void and Array<Dynamic>->Dynamic functions. This only works incidentally on some targets because the old @:overload(function) syntax doesn't perform the same checks that proper overload functions do. For example, to the JVM target both signatures are the same, so the compiler doesn't know which overload to resolve to.

Since this is only done so that we can support both Dynamic and Void callback returns, I think using a type parameter instead achieves the same goal without the overload hassle.

I had the formatter on so the diff is a bit bigger.

[Simn]

I thought this change might break cases where we rely on explicit Dynamic, for instance the mixed array thing in something like Reflect.makeVarArgs(_ -> ["a", 1]). However, this fails on development too (I'm not sure why though...)

[Simn]

A green CI on a first commit that changes all Reflect.hx files must be some sort of divine sign... I'll merge this before anyone has a chance to complain.