Add label for Block Title component to satisfy accessibility

[tjcafferkey]

Fixes #1680

Accessibility

• I've tested using only a keyboard (no mouse)
• I've tested using a screen reader
• All animations respect prefers-reduced-motion
• All text has at least a 4.5 color contrast with its background
• I've updated this doc for any feature flags or experimental interfaces implemented in this pull request.

Screenshots

How to test the changes in this Pull Request:

1. In a new page add Filter Products By Attribute (and Price) block
2. Choose an attribute and then click "Done"
3. Inspect the blocks title within the editor
4. Observe <label /> element which has for attribute value which corresponds to the <textarea /> id value

Changelog

Add label element to <BlockTitle> component

[mikejolley]

Thanks for submitting this patch. I can confirm this fixes the issue in the editor 👍🏻