generalise pattern of treating $foo as a store value but $$foo as internal

src/compile/render-dom/index.ts

@@ -294,7 +294,7 @@ export default function dom(
 		return true;
 	});
 
-	const reactive_stores = component.vars.filter(variable => variable.name[0] === '$' && variable.name !== '$$props');
+	const reactive_stores = component.vars.filter(variable => variable.name[0] === '$' && variable.name[1] !== '$');
 
 	if (renderer.slots.size > 0) {
 		const arr = Array.from(renderer.slots);

src/compile/render-ssr/index.ts

@@ -24,7 +24,7 @@ export default function ssr(
 		{ code: null, map: null } :
 		component.stylesheet.render(options.filename, true);
 
-	const reactive_stores = component.vars.filter(variable => variable.name[0] === '$' && variable.name !== '$$props');
+	const reactive_stores = component.vars.filter(variable => variable.name[0] === '$' && variable.name[1] !== '$');
 	const reactive_store_values = reactive_stores
 		.map(({ name }) => {
 			const store = component.var_lookup.get(name.slice(1));