Prevent `.` in regex

[adrianbw]

Originally suggested by @timneutkens because the Next.js team will be adding it. Periods in regex tend to lead to a lot of unintentional matching, and this will help prevent performance hits from doing so.

[mrmckeb]

Is there an ESLint rule that can help with this? I had a look, but couldn't find one.

[timneutkens]

I think we'll have to write one but shouldn't be too hard as regexes are a separate AST node.