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/doc_classes/@GDScript.xml

@@ -622,6 +622,12 @@
 				[/codeblock]
 			</description>
 		</annotation>
+		<annotation name="@static_unload">
+			<return type="void" />
+			<description>
+				Make a script with static variables to not persist after all references are lost. If the script is loaded again the static variables will revert to their default values.
+			</description>
+		</annotation>
 		<annotation name="@tool">
 			<return type="void" />
 			<description>