Document how to add a new function in godot/core that is accessible via JS binding

[Flux159]

I was just experimenting with adding a new function to godot/core/image.cpp to access through Javascript and thought it would be useful to document for others who needed new C++ methods accessible via Javascript (for perf or other reasons).

I made a gist here: https://gist.github.com/Flux159/b602806d3941e530e87cf969400a7994 with notes, but the basics are as follows for image.cpp:

• Update the header file for the function(s)
• Write the implementation for the functions in the .cpp file
• Updating the Image::_bind_methods() function with the new functions (easy to forget to to this, godot will compile, but you won't be able to access in JS without updating the bind_methods function).

Then recompile godot w/ the ECMAScript module.

I don't know if there are limitations to functions created this way (do return values need to be specific primitives?) and I don't know if other core apis always have a _bind_methods() function, but it would be helpful to add to the readme, wiki, or some other docs how to do this in a generic way.

[Flux159]

This is documented in godot's documentation & seems to work the same for Ecmascript, closing out.

https://docs.godotengine.org/en/stable/development/cpp/binding_to_external_libraries.html