doc: Add default values to all properties

Thanks to @bojidar-bg's impressive work in #29380.

modules/visual_script/doc_classes/VisualScriptVariableSet.xml

@@ -16,7 +16,7 @@
 	<methods>
 	</methods>
 	<members>
-		<member name="var_name" type="String" setter="set_variable" getter="get_variable">
+		<member name="var_name" type="String" setter="set_variable" getter="get_variable" default="&quot;&quot;">
 			The variable's name.
 		</member>
 	</members>