Support :file-pattern on :ns-group

[ericdallo]

Is your feature request related to a problem? Please describe.
At Nubank, we want to use the discouraged-var linter only on test namespaces, the issue is that we can know for sure a ns is in a test source-path only checking the ns, we need to check the source-path.

Describe the solution you'd like
if we support a :file-pattern on the :ns-group we would be able to exclude specific source-paths.

Describe alternatives you've considered
no alternatives AFAICS

Additional context
https://clojurians.slack.com/archives/CHY97NXE2/p1661880291392359

I can help with a PR

[borkdude]

Sounds good!