link-copy per Rudy

Adds a "Copy Link Address" context menu item in firefox for links, and makes it accessible with hotkey "A"

Mozilla changed the longstanding "Copy Link Address" hotkey from "A" to "L" in Firefox 88.

They have decided to not consider reverting this change, even though "L" is far less accessible than "A" and brings it out of sync with accessibility keys across other browsers.

Reference: https://bugzilla.mozilla.org/show_bug.cgi?id=1701324

Source code: https://github.com/rfaile313/link-copy