Add Cobol support (lang filters, codeintel)

Created by: dadlerj

Requested by https://app.hubspot.com/contacts/2762526/company/2428850551 (now a customer, but not urgent)

• Looks like we don't have a Cobol basic code intel extension: https://sourcegraph.com/extensions?query=cobol

• And just doing a search shows we don't provide a lang: filter option for them.

• And finally, I don't see syntax highlighting: https://sourcegraph.com/github.com/leachim6/hello-world@c46bbf3c38b3b98a78ee1bd38b89f4a888e271bb/-/blob/c/cobol.cbl

I believe the file extensions are .cbl or .cob or .cpy

@christinaforney Am I missing any steps here? it would be awesome if we had a checklist/process in the dev handbook for "how to add a language"! I have no idea who to file that issue on though :)