chore(sfc-playground): default isCustomElement for sfc playground

packages/sfc-playground/src/App.vue

@@ -54,7 +54,8 @@ const sfcOptions = computed(
     template: {
       isProd: productionMode.value,
       compilerOptions: {
-        isCustomElement: (tag: string) => tag === 'mjx-container',
+        isCustomElement: (tag: string) =>
+          tag === 'mjx-container' || tag.startsWith('custom-'),
       },
     },
   }),