build: add mjs extension to lint-js

This matches what the lint-js make target passes through the CLI.

Backport-PR-URL: #32610
PR-URL: #32145
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

tools/lint-js.js

@@ -1,7 +1,7 @@
 'use strict';
 
 const rulesDirs = ['tools/eslint-rules'];
-const extensions = ['.js', '.md'];
+const extensions = ['.js', '.mjs', '.md'];
 // This is the maximum number of files to be linted per worker at any given time
 const maxWorkload = 60;