Add comprehensive tests for parseConfiguration function, handle non-object parser results consistently (fixes #1523).

lib/markdownlint.mjs

@@ -1183,6 +1183,7 @@ export function readConfigSync(file, parsers, fs) {
   // Try to parse file
   const { config, message } = parseConfiguration(file, content, parsers);
   if (!config) {
+    // @ts-ignore
     throw new Error(message);
   }
   // Extend configuration