Fix non userdata const reference overloads

[kunitoki]

This pull request fixes a compile-time error in LuaBridge that occurred when using references or pointers to non-userdata types (like int, double, or std::string) with luabridge::overload<const T&>(...). The main change is making the isInstance method in several StackOpSelector specializations a template, allowing correct instantiation and use with various types. New tests have been added to ensure these cases work as expected.

Core Fixes to Type Handling:

• Made isInstance a template method in all relevant StackOpSelector specializations for pointers and references to non-userdata types in both Distribution/LuaBridge/LuaBridge.h and Source/LuaBridge/detail/Stack.h, allowing proper compilation and usage with types like int, double, and std::string. [1] [2] [3] [4] [5] [6] [7] [8]

Testing Improvements:

• Added ReferenceAndPointerToNonUserdataTypes test in StackTests.cpp to verify that isInstance works for references and pointers to non-userdata types, and that explicit template arguments are handled correctly.
• Added tests in OverloadTests.cpp to ensure overloads with const reference parameters for non-userdata types and strings work without ambiguity or compile errors.