Bug 1743047 - Use "reveal" rather than show terminology consistently for the reveal password button. r=Gijs,fluent-reviewers

The pseudo-class and nsContextMenu context attribute were using reveal,
the pseudo-element and webidl attribute were using "show".

Use reveal consistently and update the accesskey so that there aren't
conflicts with existing commands. Also enable the feature in
browser_contextmenu_input.js so that this change is tested.

Differential Revision: https://phabricator.services.mozilla.com/D136086