Allow to define more than one subgroup level in shader material

[Chaosus]

• Closes Increase shader subgroup nesting depth godot-proposals#14609

Test code:

shader_type canvas_item;

group_uniforms Foo.test.Cool;
uniform float t; 

group_uniforms Foo.test;
uniform float aa;

group_uniforms Foo;
uniform vec3 v;

group_uniforms;
uniform vec3 vv;

group_uniforms Foo2;
uniform vec3 v2;

group_uniforms Foo.test.Cool;
uniform float v3; 

group_uniforms;
uniform vec3 vv4;

void fragment() {
}