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_variables_other.gd

@@ -0,0 +1,11 @@
+@static_unload
+class_name StaticVariablesOther
+
+static var perm := 0
+
+static var prop := "Hello!":
+	get: return prop + " suffix"
+	set(value): prop = "prefix " + str(value)
+
+func test():
+	print("ok")