chore: add reproduction in playground

Author: atinux

playground/components/mdc/Callout.vue

@@ -0,0 +1,5 @@
+<template>
+  <div style="border: 1px solid #ddd; padding: 1rem;">
+    <MDCSlot unwrap="p" />
+  </div>
+</template>

playground/components/mdc/List.vue

@@ -0,0 +1,5 @@
+<template>
+  <div>
+    <MDCSlot :use="$slots.default" unwrap="ul li" />
+  </div>
+</template>

playground/pages/unwrap.vue

@@ -3,6 +3,17 @@ const mdList = `
 - Hello
 - World
 `
+
+const mdc = `
+::callout
+This is a callout
+::
+
+::list
+- Hello
+- World
+::
+`
 </script>
 
 <template>
@@ -18,5 +29,7 @@ const mdList = `
     <MDC unwrap="ul" :value="mdList" />
     <div>Unwrap `ul li`:</div>
     <MDC unwrap="ul li" :value="mdList" />
+    <hr />
+    <MDC :value="mdc" />
   </div>
 </template>