GDScript: Initialize all defaults beforehand in implicit constructor

Set all the default values for typed variables before actually trying to
initialize them, including `@onready` ones.

This ensures that if validated calls are being used there will be a
value of the correct type, even if the resolution is done out of order
or deferred because of `@onready`.

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

@@ -0,0 +1,20 @@
+extends Node
+
+@onready var later_inferred := [1]
+@onready var later_static : Array
+@onready var later_static_with_init : Array = [1]
+@onready var later_untyped = [1]
+
+func test():
+	assert(typeof(later_inferred) == TYPE_ARRAY)
+	assert(later_inferred.size() == 0)
+
+	assert(typeof(later_static) == TYPE_ARRAY)
+	assert(later_static.size() == 0)
+
+	assert(typeof(later_static_with_init) == TYPE_ARRAY)
+	assert(later_static_with_init.size() == 0)
+
+	assert(typeof(later_untyped) == TYPE_NIL)
+
+	print("ok")

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

@@ -0,0 +1,2 @@
+GDTEST_OK
+ok