RichText: Add a method to manage focus

[afercia]

Looks like with the removal of unstableOnSetup in #10744 there isn't a built-in way to manage focus on the RichText editable area any longer.

Managing focus is essential in some flows, especially for custom block types with multiple RichText instances. While the removal of unstableOnSetup was expected to happen without notice, a new, alternative, method was expected as well.

Other editors expose a focus method: for example, Draft.js explains very well why this is needed, see https://draftjs.org/docs/advanced-topics-managing-focus

Discussed a bit this issue with our technical team at Yoast and we'd tend to think that also all use-cases that were offered by a reference to TinyMCE would need to be offered by the wrapper, RichText in this case.

[atimmer]

I think this is essential functionality for an editable field component, which RichText is. I understand the choice to not expose the TinyMCE instance, but focusing an editable field is core functionality.

[ellatrix]

What's the difference with #9740?

[afercia]

The difference is I forgot #9740 existed 🙂