feat(Form): improve form control and input validation trigger (#487)

Co-authored-by: Benjamin Canac <canacb1@gmail.com>

docs/components/content/examples/FormGroupErrorExample.vue

@@ -0,0 +1,9 @@
+<template>
+  <UFormGroup v-slot="{ error }" label="Email" :error="!email && 'You must enter an email'" help="This is a nice email!">
+    <UInput v-model="email" type="email" placeholder="Enter email" :trailing-icon="error && 'i-heroicons-exclamation-triangle-20-solid'" />
+  </UFormGroup>
+</template>
+
+<script setup lang="ts">
+const email = ref('')
+</script>