Support __call on class type vars

[JohnnyMorganz]

Currently, the metatable of a class type var is not correctly checked to see if it is callable (i.e. has a __call metatable).
We resolve this issue by checking the metatable in checkCallOverload

Fixes #756

[vegorov-rbx]

Can you please add a flag for new if block?

It will be enough to have something like:
else if (const ClassTypeVar* ctv = get<ClassTypeVar>(fn); FFlag::LuauCallableClasses && ctv && ctv->metatable)

With a flag definition at the start of the file like: LUAU_FASTFLAGVARIABLE(LuauCallableClasses, false).

And a ScopedFastFlag luauCallableClasses{"LuauCallableClasses", true}; at the start of the test.