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 Its README says traffic leaves from unpublished addresses in those networks.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/lists.go

@@ -146,6 +146,30 @@ 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: "https://raw.githubusercontent.com/tn3w/ProtonVPN-IPs/master/" +
+				"protonvpn_ip_ranges.txt",
+			Interval: vpnListInterval,
+			Format:   FormatProtonVPN,
+			VPN:      true,
+		},
+		{
+			Name:        "protonvpn_entry",
+			Description: "A server of the ProtonVPN service",
+			URL: "https://raw.githubusercontent.com/tn3w/ProtonVPN-IPs/master/" +
+				"protonvpn_entry_ips.txt",
+			Interval: vpnListInterval,
+			Format:   FormatProtonVPN,
+			VPN:      true,
+		},
 		{
 			Name: "x4bnet",
 			Description: "In an IPv4 network of a VPN service, according to " +