chore(SelectMenu): handle multiple default display + specific placeholder

docs/components/content/examples/SelectMenuExampleMultipleSlot.vue

@@ -0,0 +1,14 @@
+<script setup>
+const people = ['Wade Cooper', 'Arlene Mccoy', 'Devon Webb', 'Tom Cook', 'Tanya Fox', 'Hellen Schmidt', 'Caroline Schultz', 'Mason Heaney', 'Claudie Smitham', 'Emil Schaefer']
+
+const selected = ref([])
+</script>
+
+<template>
+  <USelectMenu v-model="selected" :options="people" multiple>
+    <template #label>
+      <span v-if="selected.length" class="truncate">{{ selected.join(', ') }}</span>
+      <span v-else>Select people</span>
+    </template>
+  </USelectMenu>
+</template>