feat(Modal): add prevent-close prop

Resolves #303

docs/components/content/examples/ModalExamplePreventClose.vue

@@ -0,0 +1,24 @@
+<script setup>
+const isOpen = ref(false)
+</script>
+
+<template>
+  <div>
+    <UButton label="Open" @click="isOpen = true" />
+
+    <UModal v-model="isOpen" prevent-close>
+      <UCard :ui="{ ring: '', divide: 'divide-y divide-gray-100 dark:divide-gray-800' }">
+        <template #header>
+          <div class="flex items-center justify-between">
+            <h3 class="text-base font-semibold leading-6 text-gray-900 dark:text-white">
+              Modal
+            </h3>
+            <UButton color="gray" variant="ghost" icon="i-heroicons-x-mark-20-solid" class="-my-1" @click="isOpen = false" />
+          </div>
+        </template>
+
+        <Placeholder class="h-32" />
+      </UCard>
+    </UModal>
+  </div>
+</template>