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/parse.go

@@ -58,6 +58,8 @@ func parse(r io.Reader, format Format) (*netipx.IPSet, error) {
 		return parsePIA(r)
 	case FormatIVPNJSON:
 		return parseIVPN(r)
+	case FormatProtonVPN:
+		return parseProtonVPN(r)
 	default:
 		return nil, errUnknownFormat
 	}
@@ -329,6 +331,22 @@ func parseIVPN(r io.Reader) (*netipx.IPSet, error) {
 	return addressSet(texts)
 }
 
+// parseProtonVPN returns the addresses or networks in a ProtonVPN-IPs list. A
+// file with no addresses or networks fails, because the service always has
+// servers.
+func parseProtonVPN(r io.Reader) (*netipx.IPSet, error) {
+	addresses, err := parseLines(r, addAddressOrNetwork)
+	if err != nil {
+		return nil, err
+	}
+
+	if len(addresses.Ranges()) == 0 {
+		return nil, errNoAddresses
+	}
+
+	return addresses, nil
+}
+
 // readJSON reads r to its end and decodes it, as one JSON document, into v.
 func readJSON(r io.Reader, v any) error {
 	data, err := io.ReadAll(r)