A11y: Implement live regions to announce when autocomplete liquid listbox opens and when navigating options

[RachelRVasquez]

To be merged after PR #3701

All Submissions:

• Have you followed the Contributing guideline?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Implemented live regions so when using VoiceOver/NVDA, the autocomplete liquid listbox announces itself on open/close, as well as the options as you navigate via keyboard.

How to test the changes in this Pull Request:

1. Test using VoiceOver + Safari and/or NVDA + Firefox/Chrome.
2. On your WordPress admin, go to Memberships > Settings > Email Templates. Select any email template to test with.
3. Click or enter the "Body" TinyMCE field with the tab key.
4. Type a left curly brace {. The autocomplete box should appear. Does NVDA announce it?
5. Navigate the options with your arrow keys. Does NVDA read out each option?
6. Test the NVDA also announces when the autocomplete box closes (hit the esc key to keyboard test).

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?

Changelog entry

Added live regions so that the autocomplete listbox for liquid announces itself when it appears, announces when it's closed and announces the options as you navigate via the arrow keys via NVDA/assistive technologies.