fix: update test config to import from dist instead of deleted index.mjs

dev-server-import-maps test config referenced ../index.mjs which was
deleted. Import from ../dist/index.js instead.

Assisted-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: bennypowers

packages/dev-server-import-maps/test-browser/web-test-runner.config.mjs

@@ -1,6 +1,6 @@
 import { join } from 'path';
 import { fileURLToPath } from 'url';
-import { importMapsPlugin } from '../index.mjs';
+import { importMapsPlugin } from '../dist/index.js';
 
 export default {
   rootDir: fileURLToPath(join(import.meta.url, '..', '..', '..', '..')),