Extension publish command should warn user if repository field not set in package.json

[ryan-blunden]

The goal is to get as many extensions as possible with a repository object set in their package.json with the assumption that a legitimate extension will have a git repository.

One way to achieve this is to set it when the extension is bootstrapped by the extension creator (https://github.com/sourcegraph/create-extension/issues/29) but another way is to prompt them with a message upon publishing.

I'm proposing that src extension publish would detect if the repository field was missing and display a message asking them to add the repository field in the future. It would not block the publishing process.