Handle 'bitcoin:<address>' links (BIP21)

It would be neat that, when clicking (in a browser) on a BIP21 link, Specter would pop up with a prefilled transaction. I've briefly look into how Electrum is doing it and it doesn't seem too hard(?).

On Linux a new mime type needs to be registered: https://github.com/spesmilo/electrum/blob/master/electrum.desktop#L17
MacOSX and Windows have something similar: https://github.com/spesmilo/electrum/blob/master/contrib/osx/make_osx#L167-L171 resp https://github.com/spesmilo/electrum/blob/master/contrib/build-wine/electrum.nsi#L135-L139

I can even imagine Electron handling this in a multi platform way.