This is a pretty simple implementation of the approach discussed in the bug. When a tab-to-search result is shown, we announce it to screen readers. We keep track of which engines are announced to be sure we don't keep announcing the same engine as the user types.

This approach is maybe slightly crude, but I didn't find a working elegant approach. One approach I considered is checking for tab-to-search results in the loop in UrlbarView._updateResults. After the loop is finished, if we found a tab-to-search result, we go back and change the aria-label on the action text of the prior result to something like "Visit, or press Tab to search with { $engine }. This didn't work because we don't read action text for the heuristic result as the user types. This is presumbably to avoid disrupting the user as they type. This is what my patch does regardless, however... Since Jamie was skeptical about whether we should announce this at all, I put this behaviour behind a default-true pref, browser.urlbar.tabToSearch.accessibility.announceResults.

Differential Revision: https://phabricator.services.mozilla.com/D93451