Allow GDExtensions to register virtual methods and call them on scripts

test/project/example.gd

@@ -0,0 +1,5 @@
+extends Example
+
+func _do_something_virtual(p_name, p_value):
+	custom_signal.emit(p_name, p_value)
+	return "Implemented"

test/project/main.gd

@@ -241,6 +241,10 @@ func _ready():
 	assert_equal(new_example_ref.was_post_initialized(), true)
 	assert_equal(example.test_post_initialize(), true)
 
+	# Test a virtual method defined in GDExtension and implemented in script.
+	assert_equal(example.test_virtual_implemented_in_script("Virtual", 939), "Implemented")
+	assert_equal(custom_signal_emitted, ["Virtual", 939])
+
 	exit_with_status()
 
 func _on_Example_custom_signal(signal_name, value):

test/project/main.tscn

@@ -1,11 +1,13 @@
-[gd_scene load_steps=2 format=3 uid="uid://dmx2xuigcpvt4"]
+[gd_scene load_steps=3 format=3 uid="uid://dmx2xuigcpvt4"]
 
 [ext_resource type="Script" path="res://main.gd" id="1_qesh5"]
+[ext_resource type="Script" path="res://example.gd" id="2_jju25"]
 
 [node name="Node" type="Node"]
 script = ExtResource("1_qesh5")
 
 [node name="Example" type="Example" parent="."]
+script = ExtResource("2_jju25")
 
 [node name="ExampleMin" type="ExampleMin" parent="Example"]
 layout_mode = 0