https://mozilla-hub.atlassian.net/browse/SNT-616

This implements most parts of Pocket suggestions. They don't need any special UI
or a dynamic result type because they're only shown as the usual best match rows
or non-best match rows.

Still to do:

• Implement the "Show less frequently" behavior once we decide what the keywords
  will be and how that will work.
• Implement the bottom "Mozilla Pocket" text inside the suggestion row once it's
  finalized. We can use the same technique we use to show the "Sponsored" bottom
  text for adM suggestions.

Other changes this makes:

• Replace the type=bestmatch attribute with an is-bestmatch attribute. That
  lets best-match rows have a type too, in this case "pocket" (actually
  either "rs_pocket" or "merino_pocket", since I'm using the telemetry
  result type).
• Improve how UrlbarProviderQuickSuggest delegates to individual features when
  getting result commands, view updates, handling commands, etc., so that we
  don't need to add new case statements for each new type of suggestion.

https://hg.mozilla.org/mozilla-central/rev/da3399f91bee