display keyboard shortcuts in suggestions

[seb314]

@dj311 would you be open to a feature like this? The current implementation is not completed yet, as only a subset of shortcuts has been added to commands.json. But I'd complete that if you consider the overall approach appropriate.

Background: In my experience, this is a very effective way to learn keyboard shortcuts without a steep learning curve.

Limitations: shortcuts must be manually maintained in commands.json, but this probably works reasonably well as shortcuts probably don't change that often?

TODO: add more shortcuts from https://support.mozilla.org/en-US/kb/keyboard-shortcuts-perform-firefox-tasks-quickly to commands.js