Add support to import/export to Bitwarden

[evertonstz]

It's good for people trying to move in or move out from Bitwarden into firefox, since Bitwarden is able to export and import CSV files, implementing this should be as easy as adding a flag.

"url" column in Bitwarden's CSV is "login_uri", "username" is "login_username" and "password" is "login_password".

Be aware that Bitwarden exports more than just passwords, it also exports notes and probably some other things. All you have to do is only import to firefox the lines that have the column "type" == login.

Cheers!

[andreicristianpetcu]

Please use Firefox Nightly for now @evertonstz https://github.com/mozilla-lockwise/lockwise-addon/issues/332#issuecomment-653938961