feat(Popover): add anchor slot (#4119)

Co-authored-by: Jakub <jakub.michalek@freelo.io>
Co-authored-by: Benjamin Canac <canacb1@gmail.com>

docs/app/components/content/examples/popover/PopoverAnchorSlotExample.vue

@@ -0,0 +1,19 @@
+<script lang="ts" setup>
+const open = ref(false)
+</script>
+
+<template>
+  <UPopover
+    v-model:open="open"
+    :dismissible="false"
+    :ui="{ content: 'w-(--reka-popper-anchor-width) p-4' }"
+  >
+    <template #anchor>
+      <UInput placeholder="Focus to open" @focus="open = true" @blur="open = false" />
+    </template>
+
+    <template #content>
+      <Placeholder class="w-full aspect-square" />
+    </template>
+  </UPopover>
+</template>