Return the scheme
This function returns a value which depends on the scheme in the url:
assert( url_view( "wss://www.example.com/crypto.cgi" ).scheme_id() == scheme::wss );
URI = scheme ":" hier-part [ "?" query ] [ "#" fragment ]
absolute-URI = scheme ":" hier-part [ "?" query ]
scheme = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )
has_scheme , scheme .