Add browser extension support for pull request discussion code views in GitHub

[erzhtor]

Follow-up on https://github.com/sourcegraph/sourcegraph/pull/30618.

While adding support for pull request commit view @taras-yemets and I noticed that we don't support pull requests discussions view.

[github-actions]

Heads up @muratsu @rrhyne @jjinnii - the "team/growth-and-integrations" label was applied to this issue.