[FIX] Fix handling of log level "silent"

npmlog already supports this level. We just forgot to add it to our
list, causing errors like "Failed to find current log level
"silent" in list of expected log levels" when checking for enabled log
levels.

lib/logger.js

@@ -1,6 +1,6 @@
 const npmlog = require("npmlog");
 
-const levels = ["silly", "verbose", "info", "warn", "error"];
+const levels = ["silly", "verbose", "info", "warn", "error", "silent"];
 if (process.env.UI5_LOG_LVL) {
 	const logLvl = process.env.UI5_LOG_LVL;
 	if (!levels.includes(logLvl)) {
@@ -129,6 +129,9 @@ module.exports = {
 	},
 
 	setLevel(level) {
+		if (!levels.includes(level)) {
+			throw new Error(`Unkown log level "${level}"`);
+		}
 		return npmlog.level = level;
 	},