This allows users to set
`programs.firefox.nativeMessagingHosts.euwebid = true` to make the
native companion app available in Firefox.
It's used by the Web eID browser extension
(https://addons.mozilla.org/en/firefox/addon/web-eid-webextension/),
which can be tried out at https://web-eid.eu/ - pressing the
"Authenticate" button there should open a new window.
