Add the ProtonVPN server networks and addresses to the IP lookup (closes #236)

Adds the VPN lists `protonvpn`, the servers' /24 networks, IPv4 only, and `protonvpn_entry`, the servers' IPv4 and IPv6 addresses. Each is downloaded once a day and shows in `flags.is_vpn` and `vpn_providers` with no other change.

Proton's `/vpn/logicals` answers 401 without a logged-in session, even with a current client's `x-pm-appversion`, so both come from the public ProtonVPN-IPs mirror. No request header is needed.

Judgement call: the mirror's /24 file as well as its exact addresses. Its README says traffic leaves from unpublished addresses in those networks. The cost is that neighbouring addresses are listed too.

Model: opus-5-5

internal/iplist/iplist.go

@@ -51,6 +51,11 @@ const (
 	// FormatIVPNJSON is IVPN's JSON server list. The hosts of its wireguard
 	// and openvpn gateways have their addresses in host and v2ray.
 	FormatIVPNJSON
+
+	// FormatProtonVPN has one address or network per line, with comment lines
+	// that start with #. Unlike FormatLines, a file with no addresses or
+	// networks fails.
+	FormatProtonVPN
 )
 
 // List is one public IP list.