Added overloads for const std::string& and const char* arguments

[Bigfoot71]

This is primarily a suggestion, wouldn't adding an overload function LoadFromMemory(const char* vsCode, const char* fsCode) be more convenient in the case where we might, for example, want to load a fragment shader from memory with the default vertex shader?

For the sake of consistency, I've also added one for static ::Shader Load(...), to see if it would be equally useful.

[RobLoach]

Thanks a lot 👍 ... Could apply to the other ones classes too, but not absolutely needed.

[Bigfoot71]

I checked just in case, but I don't see any other raylib functions that would have a specific behavior when a null pointer is given rather than a "string".

The proposed modification was primarily for this scenario.