Repeat Visitor: Add "return" as a keyword to trigger it

[kraftbj]

It's easy to try /return to get a "Returning" visitors block. Let's add that to what triggers adding the block.

Steps to reproduce the issue

1. Block Editor
2. Try adding a "Returning" visitor block.

What I expected

Return is pretty close to repeat visitor in concept, so the block should appear.

What happened instead

No block.

Screenshots

[jsnmoon]

We'd love to add this, but we've already reached the maximum of 3 keywords for this block.

For future reference, we'd also like to add the following keywords:

• hidden
• conditional