Refactor to move parseConfiguration function into a separate file, add stronger typing, add first test case.

lib/parse-configuration.mjs

@@ -0,0 +1,42 @@
+// @ts-check
+
+/**
+ * Result of a call to parseConfiguration.
+ *
+ * @typedef {Object} ParseConfigurationResult
+ * @property {Object | null} config Configuration object if successful.
+ * @property {string} message Error message if an error occurred.
+ */
+
+/**
+ * Parse the content of a configuration file.
+ *
+ * @param {string} name Name of the configuration file.
+ * @param {string} content Configuration content.
+ * @param {import("./markdownlint.mjs").ConfigurationParser[]} [parsers] Parsing function(s).
+ * @returns {ParseConfigurationResult} Parse configuration result.
+ */
+export default function parseConfiguration(name, content, parsers) {
+  let config = null;
+  let message = "";
+  const errors = [];
+  let index = 0;
+  // Try each parser
+  (parsers || [ JSON.parse ]).every((parser) => {
+    try {
+      config = parser(content);
+    } catch (error) {
+      errors.push(`Parser ${index++}: ${error.message}`);
+    }
+    return !config;
+  });
+  // Message if unable to parse
+  if (!config) {
+    errors.unshift(`Unable to parse '${name}'`);
+    message = errors.join("; ");
+  }
+  return {
+    config,
+    message
+  };
+}