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/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.