fix: improve screen reader behaviour (fixes #131, fixes #200)

[greatislander]

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• This isn't a duplicate of an existing pull request

Description

Adds screen reader feedback when a "Deselect all" button is clicked in the filter list. Announces notifications when they are added to the DOM.

Steps to test

1. Enable a screen reader (NVDA or VoiceOver).
2. Check some topics on the Browse All page's filter list.
3. Click the "Deselect all" button under topics.
4. Test all behaviours which display notifications (saving/removing saved searches, saving/removing favorites).

Expected behavior: "All topics have been deselected" is announced to screen reader users.

Additional information

Implemented using the @wordpress/a11y "speak" utility.

Related issues

• Resolves Deselect All does not provide screen reader feedback #131.
• Resolves Notification with role="alert" is not announced after performing an action in a dialog #200.