Bug 1676469 - Convert matchBuckets to a granular list of result groups and rewrite the muxer to use it. r=preferences-reviewers,mak,harry

Summary of changes:

* Replace the `matchBuckets` pref with `resultBuckets`.
* `resultBuckets` stores a JSON'ed root result bucket object. See the big
  javadoc above `makeResultBuckets` in UrlbarPrefs for details.
* Convert existing result groups to more granular groups. This lets us remove
  the heuristic-ordering logic in the muxer. It also lets us fix a subtle
  inconsistency with regard to remote suggestions and form history; see the
  removal of the two comments that start with "Note that the second form history
  result appears after the remote suggestions" and "Note that the remote
  suggestions appear in between the two form history results" in
  test_search_suggestions.js.
* Unlike `matchBuckets`, `resultBuckets` stores *all* buckets, including the
  heuristic at the beginning. Modify the muxer so that it discards heuristic
  results that are not the first result.

Depends on D103137

Differential Revision: https://phabricator.services.mozilla.com/D99281

Author: 0c0w3

browser/app/profile/firefox.js

@@ -1258,8 +1258,8 @@ pref("services.sync.prefs.sync.browser.tabs.loadInBackground", true);
 pref("services.sync.prefs.sync.browser.tabs.warnOnClose", true);
 pref("services.sync.prefs.sync.browser.tabs.warnOnOpen", true);
 pref("services.sync.prefs.sync.browser.taskbar.previews.enable", true);
-pref("services.sync.prefs.sync.browser.urlbar.matchBuckets", true);
 pref("services.sync.prefs.sync.browser.urlbar.maxRichResults", true);
+pref("services.sync.prefs.sync.browser.urlbar.resultBuckets", true);
 pref("services.sync.prefs.sync.browser.urlbar.showSearchSuggestionsFirst", true);
 pref("services.sync.prefs.sync.browser.urlbar.suggest.bookmark", true);
 pref("services.sync.prefs.sync.browser.urlbar.suggest.history", true);