feat(Modal): open programmatically (#78)

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

playground/components/ModalProgrammaticExample.vue

@@ -0,0 +1,18 @@
+<script lang="ts" setup>
+const modal = useModal()
+
+defineProps<{
+  count: number
+}>()
+</script>
+
+<template>
+  <UModal title="This modal was opened programmatically">
+    <template #footer>
+      <div class="flex w-full justify-between">
+        <p>Count: {{ count }}</p>
+        <UButton color="gray" label="Close" @click="modal.close()" />
+      </div>
+    </template>
+  </UModal>
+</template>