fix(datasource/npm): expose default registry urls (#29717)

lib/modules/datasource/npm/index.ts

 import * as npmVersioning from '../../versioning/npm';
 import { Datasource } from '../datasource';
 import type { GetReleasesConfig, ReleaseResult } from '../types';
+import { defaultRegistryUrls as npmDefaultRegistryUrl } from './common';
 import { getDependency } from './get';
 
 export { setNpmrc } from './npmrc';
@@ -14,6 +15,8 @@ export class NpmDatasource extends Datasource {
 
   override readonly defaultVersioning = npmVersioning.id;
 
+  override readonly defaultRegistryUrls = npmDefaultRegistryUrl;
+
   override readonly releaseTimestampSupport = true;
   override readonly releaseTimestampNote =
     'The release timestamp is determined from the `time` field in the results.';