Add protocol buffers to our lang: search filter

Created by: dadlerj

Requested by https://app.hubspot.com/contacts/2762526/company/768958891

We were screensharing, and the user seemed concerned that Sourcegraph didn't support searching for proto files... I wonder if there's a downside of having the lang filter, in that not having everything listed there erodes confidence? This is almost certainly overthinking it, but wanted to share (CC @christinaforney)

.proto and .pb seem to be the relevant extensions here...