Allow using stage functions inside custom shader functions

[Chaosus]

This is to allow SDF functions (in canvas_item) as well as emit_particle (in particle shaders) to be used in custom shader functions. Closes godotengine/godot-proposals#10047

[Calinou]

Tested locally (rebased on top of master 44fa552), it works as expected.

Previously, this shader would have failed compilation:

shader_type particles;

void create_particles() {
	emit_subparticle(mat4(1.0), vec3(1.0), vec4(1.0), vec4(1.0), uint(0));
}

void process() {
	create_particles();
}