VSCE: Ability to open repository or file location in code host

Created by: jjinnii

Feature request description

As a user, I would like to easily navigate to a repository's page on Github/Gitlab, so that I can learn more about the repository and/or view the repository in a more familiar format. As a user, I would like to easily navigate to a file location on Github/Gitlab, so that I can view code in a more familiar format and/or make use of other code host extensions that I have on my code host.

Additional context

In Sourcegraph, we have the Github extension that allows user to click "View on Github". I'm not entirely sure if it's possible for us to integrate Sourcegraph extensions into our VSCE extension. (Would we even want to? I feel like it'd be confusing to have extensions for an extension) Before tackling this problem, some technical guidance is needed on Sourcegraph extension in VSCE.