Module support

[ashtonmeuser]

Adapted the project to compile as either an addon or module. Closes #7. Does not actually allow for web/HTML5 exports due to Wasmer not being able to be compiled to Wasm via Emscripten (which is what happens internally when compiling Godot export templates). More details in #18).

Surprisingly few compromises and defines were required. An overview of the quirks introduced:

1. GODOT_MODULE define determines how Godot sources are included. To make things easier, all Godot source includes should go in defs.h. While a little awkward, I think this is an okay pattern for the (currently) very limited imports used. If this proves problematic, could use the same pattern in each individual source file. Example:

   #ifdef GODOT_MODULE // Godot includes when building module
     #include "scene/2d/sprite.h"
   #else // Godot addon includes
     #include <Godot.hpp>
     #include <Sprite.hpp>
   #endif
   

2. Method are registered via _bind_methods in Godot and _register_methods. REGISTRATION_METHOD is used to abstract this difference. Inside the method, there's a check on the GODOT_MODULE define to actually register the methods differently. Example:

   void MyClass::REGISTRATION_METHOD() {
     #ifdef GODOT_MODULE
       ClassDB::bind_method(D_METHOD("my_method"), &MyClass::my_method);
     #else
       register_method("my_method", &MyClass::my_method);
     #endif
   }
   

3. NS define used to define the godot:: namespace that is required for addon and blank (::) namespace for module.
4. godot_error (used in GDNative) is defined as an alias for Error for modules.
5. Errors used by the addon are defined explicitly using the same name without the godot_ prefix. This aligns error names with Godot proper (3.x and 4) and GDExtension. For example, Godot and GDExtension use OK while GDNative uses GODOT_OK. Now an OK define exists to align GDNative with the others.
6. Superclass of StreamPeerWasm is now set by a define (either StreamPeer or StreamPeerGDNative). StreamPeerGDNative interface is all defines. This is hideous and will hopefully remain the ugliest part the addon/module agnosticism.

To build the engine with the Godot Wasm module, add the entire project in godot/modules/wasm and run scons platform=MY_PLATFORM.

CC @Trey2k

[Trey2k]

I think godot-library.cpp is probably rendered unnecessary by register_types.cpp, they both do pretty much the same thing. When working on the gd4 PR this was the route I went with.

[ashtonmeuser]

I think godot-library.cpp is probably rendered unnecessary by register_types.cpp

This file is wrapped if an ifdef that precludes it when used as a module. However, just to be safe, I excluded the file from the source glob as well.