Add support for static variables in GDScript

Which allows editable data associated with a particular class instead of
the instance. Scripts with static variables are kept in memory
indefinitely unless the `@static_unload` annotation is used or the
`static_unload()` method is called on the GDScript.

If the custom function `_static_init()` exists it will be called when
the class is loaded, after the static variables are set.

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

@@ -0,0 +1,13 @@
+@static_unload
+
+static var foo = "bar"
+
+static func _static_init():
+	print("static init called")
+	prints("foo is", foo)
+
+func test():
+	var _lambda = func _static_init():
+		print("lambda does not conflict with static constructor")
+
+	print("done")