Support using editor particle function "Create Emission Points From Node" in code during runtime

[RisingThumb]

Closes godotengine/godot-proposals#8106 as this functionality is very helpful to my current project.

This mostly ports over the code in editor/scene/3d/particles_3d_editor_plugin.cpp, in particular the _node_selected method, the _generate method and both GPUParticle's generate_emission_particles method and CPUParticle's generate_emission_particles method.

The only tweaks are removing any editor-only messages and a slight tweak so _node_selected is called when doing generate_emission_points.

I have also added corresponding documentation for the new generate_emission_points methods and enums and also provided a minimum test project to try out these changes:

generateEmissionPointsTest.zip

[Yarwin]

These comments are very weird – function has no documentation outside of them (albeit lack of any documentation is Godot default 🙄), they don't follow any convention/style (one has whitespace after // while another doesn't) and I find them very noisy – like, they literally tell what code is doing, without providing any additional context possible to convey efficiently via natural language (i.e. they are documenting stuff I understand, while everything else is one big black box to me).

What is semantic difference between

			// ok FINALLY get face
			Face3 face = geometry[index];

and

			Face3 face = geometry[index];

?

[RisingThumb]

They're strange, I agree, and I don't know. They come from the original EditorPlugin code here