[whatwg/html] Add sftp to registerProtocolHandler safelisted schemes (#6254)

DRANK

[SSH File Transfer Protocol](https://en.wikipedia.org/wiki/SSH_File_Transfer_Protocol) has been around for more than 2 decades with many clients out there supporting it (some [incomplete](https://en.wikipedia.org/wiki/Category:SFTP_clients) [lists](https://www.sftp.net/clients)). the html spec already allows for ssh:// & ftp://, so sftp:// seems pretty straight froward. [the IETF RFC that defines both ssh:// & sftp://](https://tools.ietf.org/html/draft-ietf-secsh-scp-sftp-ssh-uri-04#page-5).