**Description**
- Add autofill functionality to UrlbarProvider Restrict Keywords.
**Acceptance Criteria**
- There are 3 restrict keywords `@history`, `@bookmarks`, and `@tabs`
- When the user types @ followed by the beginning of the search keyword: `@h`, `@hi`, `@his`, `@hist` etc. the user should be able to autofill the rest of the search by pressing tab
- The autofill should highlight the typed letters from the user, and show the entire keyword following what the user typed. For example:
- If the user typed `@hist` , autofill should highlight what the user typed and show the rest of the keyword `@history`:
**Example steps to test `@history` :**
- set `browser.urlbar.searchRestrictKeywords.featureGate` to true
- type out the full keyword and space `@history` , address bar should show results panel with history results
- type `@h` and press right arrow, shows results panel with history results
- type `@h` and press enter, shows results panel with history results
- type `@h` and press tab, does not show panel but selects the history restrict result. When the user starts typing, it will search their history and show results panel with history results
Differential Revision: https://phabricator.services.mozilla.com/D218742
f9e8516afa