Implement omnibox API for searching from the address bar

[alvinhochun]

The omnibox API:

• https://developer.chrome.com/docs/extensions/reference/api/omnibox
• https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/omnibox

By implementing the omnibox API, it will allow users to initiate a search with the Yimtan extension from the address bar, without having to first open the search page. It can also provide a preview of the results in the form of search suggestions.

I am aware one can add a custom search on Chrome from the settings, and on Firefox by creating a bookmark with a custom search keyword. Still, having built-in support will be more convenient, and you can do more things with the search suggestions.

_{(Unfortunately, neither works on Firefox Android for the time being: https://bugzilla.mozilla.org/show_bug.cgi?id=1427501 https://bugzilla.mozilla.org/show_bug.cgi?id=1900568)}