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

Proton's own server list answers only a logged-in session, so the lists come from the public mirror https://github.com/tn3w/ProtonVPN-IPs, downloaded once a day: `protonvpn`, its /24 networks, and `protonvpn_entry`, its servers' IPv4 and IPv6 addresses. Both feed `flags.is_vpn` and `vpn_providers`.

Judgement call: the /24 networks are listed as well as the exact addresses, because coverage matters more than attribution, so neighbouring addresses in those networks count too.
Risk: the mirror is one person's repository; if it stops updating, downloads keep succeeding with old data.

Model: opus-5-5

internal/iplist/lists.go

@@ -8,6 +8,9 @@ import (
 // TorExitsList is the name of the list of Tor exit nodes.
 const TorExitsList = "tor_exits"
 
+// protonVPNIPsURL is where the ProtonVPN-IPs mirror publishes its files.
+const protonVPNIPsURL = "https://raw.githubusercontent.com/tn3w/ProtonVPN-IPs/master/"
+
 // How often each list is downloaded. No list is downloaded more often than
 // its publisher updates it or allows.
 const (
@@ -146,6 +149,28 @@ func vpnLists() []List {
 			Format:      FormatPIAJSON,
 			VPN:         true,
 		},
+		{
+			// Proton's own server list needs a logged-in session, so this is
+			// the public ProtonVPN-IPs mirror of it. Proton publishes only
+			// the addresses clients connect to. The mirror's README says
+			// traffic leaves from others in the same /24, so this file of
+			// /24 networks is the one it says to use for detecting exits.
+			Name: "protonvpn",
+			Description: "In a /24 network that holds a server of the " +
+				"ProtonVPN service",
+			URL:      protonVPNIPsURL + "protonvpn_ip_ranges.txt",
+			Interval: vpnListInterval,
+			Format:   FormatProtonVPN,
+			VPN:      true,
+		},
+		{
+			Name:        "protonvpn_entry",
+			Description: "A server of the ProtonVPN service",
+			URL:         protonVPNIPsURL + "protonvpn_entry_ips.txt",
+			Interval:    vpnListInterval,
+			Format:      FormatProtonVPN,
+			VPN:         true,
+		},
 		{
 			Name: "x4bnet",
 			Description: "In an IPv4 network of a VPN service, according to " +