Add regexp/no-potentially-useless-backreference rule

[ota-meshi]

No description provided.

[RunDevelopment]

I must have really been sleep-deprived when I wrote the docs for this rule...

I left you a few suggestions to fix my mistakes but I couldn't think of a better description. The current rule description doesn't really convey what the rule is about IMO.

Also, I think we need a different name for this rule. I named no-potentially-empty-backreference because there was the no-empty-backreference rule. However, we have a no-useless-backreference rule instead. But I don't think that the name no-potentially-useless-backreference would be good either...

[ota-meshi]

Thank you for your review!
I changed the rule name and made some changes.

I don't think the descriptions in the documentation is confusing to the user. Even if the user does not understand the description, I think that the user will understand it because there is example code.