Fix @-mentions UI to allow @ character in posts

[guarani]

Addresses WordPress/gutenberg#24425
Related WordPress/gutenberg#24428
Related wordpress-mobile/gutenberg-mobile#2536

To test: See WordPress/gutenberg#24428 (comment)

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can test the changes on this Pull Request by downloading it from AppCenter here with build number: 32609. IPA is available here. If you need access to this, you can ask a maintainer to add you.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[hypest]

I tried build 32609 from Appcenter:

1. Typed a "@" at the start of a block, mentions UI popped up, still in "loading" state since it was from a fresh install, I tapped outside of the UI while still "loading" and the UI dismissed and the "@" was there in the block ✅
2. Typed a "@" at the start of a block, mentions UI popped up filled with many items, typed backspace and the UI dismissed leaving a "@" in the block ✅
3. Triggered the mentions UI, typed a few more chars after the "@" while in "loading" state, tapped outside to dismiss and it was only a "@" left in the block ✅
4. Triggered the UI via the toolbar button, tapped on the empty space while the UI was in "loading" state and returned to the block without the "@" being there ✅
5. Triggered the UI via the toolbar button, backspaced to delete the "@" and the UI dismissed without the "@" staying in the block ✅

So, functionally this looks good to me!

I'll have a look at the diff as well.
Edit: in the current state, the PR brings in more changes from the editor release that we don't want to ship to 15.4 yet. So, this PR will need updating to use a hotfix version (1.31.1) on the editor side.

[hypest]

Headsup that @ceyhun will be adding effort directly on this fix PRs so we make it on time for the apps releases.

[etoledom]

Approved via WordPress/gutenberg#24428 (review)

Tested after the latest update (to merge to the release branch).

[ceyhun]

Closing in favor of #14599