feat(cli): implement a default "loader" for JavaScript files

Supports any of the `.js`, `.cjs` and `.mjs` file extensions

packages/cli/src/plugin-loader-js.ts

@@ -0,0 +1,44 @@
+import { type LoaderPlugin } from '@emigrate/plugin-tools/types';
+
+const loaderJs: LoaderPlugin = {
+  loadableExtensions: ['.js', '.cjs', '.mjs'],
+  async loadMigration(migration) {
+    const migrationModule: unknown = await import(migration.filePath);
+
+    if (typeof migrationModule === 'function') {
+      return async () => {
+        await migrationModule();
+      };
+    }
+
+    if (
+      migrationModule &&
+      typeof migrationModule === 'object' &&
+      'default' in migrationModule &&
+      typeof migrationModule.default === 'function'
+    ) {
+      const migrationFunction = migrationModule.default;
+      return async () => {
+        await migrationFunction();
+      };
+    }
+
+    if (
+      migrationModule &&
+      typeof migrationModule === 'object' &&
+      'up' in migrationModule &&
+      typeof migrationModule.up === 'function'
+    ) {
+      const migrationFunction = migrationModule.up;
+      return async () => {
+        await migrationFunction();
+      };
+    }
+
+    return async () => {
+      throw new Error(`Migration file does not export a function: ${migration.relativeFilePath}`);
+    };
+  },
+};
+
+export default loaderJs;