Cop idea: Out of range referencing for Regexp

[pocke]

Is your feature request related to a problem? Please describe.

When a reference for Regepx is out of range, it always returns nil.

/(foo)bar/ =~ 'foobar'
# ok
p $1 # => foo
# bad - $2 always returns nil.
p $2 # => nil

Describe the solution you'd like

Add a cop to detect the problem.

Describe alternatives you've considered

Nothing

Additional context

We will be able to the cop with regexp_parser gem. ref: #7746