Reuse NcAutoCompleteResult, remove code duplication

[mejo-]

Describe the bug
Instead of shipping our own copy of NcAutoCompleteResult as components/Mention/AutoCompleteResult.vue, we should reuse the upstream one from @nextcloud/vue. Currently it's not possible as @nextcloud/vue doesn't export NcAutoCompleteResult.

@luka-nextcloud

[juliusknorr]

Pending release for the exported components: nextcloud-libraries/nextcloud-vue#3480 (review)