feat(linter): implement oxlint.config.ts support

apps/oxlint/src-js/js_config.ts

@@ -1,3 +1,16 @@
+import { getErrorMessage } from "./utils/utils.ts";
+import { JSONStringify } from "./utils/globals.ts";
+
+interface JsConfigResult {
+  path: string;
+  config: unknown; // Will be validated as Oxlintrc on Rust side
+}
+
+type LoadJsConfigsResult =
+  | { Success: JsConfigResult[] }
+  | { Failures: { path: string; error: string }[] }
+  | { Error: string };
+
 /**
  * Load JavaScript config files in parallel.
  *
@@ -7,6 +20,47 @@
  * @param paths - Array of absolute paths to oxlint.config.ts files
  * @returns JSON-stringified result with all configs or error
  */
-export async function loadJsConfigs(_paths: string[]): Promise<string> {
-  throw new Error("Not implemented yet");
+export async function loadJsConfigs(paths: string[]): Promise<string> {
+  try {
+    const results = await Promise.allSettled(
+      paths.map(async (path): Promise<JsConfigResult> => {
+        const fileUrl = new URL(`file://${path}`);
+        const module = await import(fileUrl.href);
+        const config = module.default;
+
+        if (config === undefined) {
+          throw new Error(`Configuration file has no default export.`);
+        }
+
+        if (typeof config !== "object") {
+          throw new Error(`Configuration file must have a default export that is an object.`);
+        }
+
+        return { path, config };
+      }),
+    );
+
+    const successes: JsConfigResult[] = [];
+    const errors: { path: string; error: string }[] = [];
+
+    for (let i = 0; i < results.length; i++) {
+      const result = results[i];
+      if (result.status === "fulfilled") {
+        successes.push(result.value);
+      } else {
+        errors.push({ path: paths[i], error: getErrorMessage(result.reason) });
+      }
+    }
+
+    // If any config failed to load, report all errors
+    if (errors.length > 0) {
+      return JSONStringify({ Failures: errors } satisfies LoadJsConfigsResult);
+    }
+
+    return JSONStringify({ Success: successes } satisfies LoadJsConfigsResult);
+  } catch (err) {
+    return JSONStringify({
+      Error: getErrorMessage(err),
+    } satisfies LoadJsConfigsResult);
+  }
 }