GDScript: Call implicit ready on base script first

It is generally expected that the base class is called before the
inherited clas. This commit implements this behavior for the implicit
ready function (`@onready` annotation) to make it consistent with the
expectations.

modules/gdscript/tests/scripts/runtime/features/onready_base_before_subclass.gd

@@ -0,0 +1,18 @@
+#GH-63329
+class A extends Node:
+	@onready var a := get_value("a")
+
+	func get_value(var_name: String) -> String:
+		print(var_name)
+		return var_name
+
+class B extends A:
+	@onready var b := get_value("b")
+
+	func _ready():
+		pass
+
+func test():
+	var node := B.new()
+	node._ready()
+	node.free()

modules/gdscript/tests/scripts/runtime/features/onready_base_before_subclass.out

@@ -0,0 +1,3 @@
+GDTEST_OK
+a
+b