feat(FormGroup): add slots (#714)

Co-authored-by: Romain Hamel <rom.hml@gmail.com>
Co-authored-by: Benjamin Canac <canacb1@gmail.com>
Co-authored-by: saveliy <savelii.moshkota@ext.jumingo.com>

docs/components/content/examples/FormGroupErrorSlotExample.vue

@@ -0,0 +1,16 @@
+<template>
+  <UFormGroup label="Email" :error="!email && 'You must enter an email'" help="This is a nice email!">
+    <template #default="{ error }">
+      <UInput v-model="email" type="email" placeholder="Enter email" :trailing-icon="error ? 'i-heroicons-exclamation-triangle-20-solid' : undefined" />
+    </template>
+
+    <template #error="{ error }">
+      <UAlert v-if="error" icon="i-heroicons-exclamation-triangle-20-solid" :title="error" color="red" />
+      <UAlert v-else icon="i-heroicons-check-circle-20-solid" title="Your email is valid" color="green" />
+    </template>
+  </UFormGroup>
+</template>
+
+<script setup lang="ts">
+const email = ref('')
+</script>