Update lint config for .mjs files

Update lint config for .mjs files and remove unused variables in
gulpfile.mjs.

.eslintrc

@@ -19,7 +19,7 @@
   },
   "overrides": [
     {
-      "files": "rollup*.js",
+      "files": ["*.mjs"],
       "env": { "node": true },
       "parserOptions": { "sourceType": "module" }
     }

gulpfile.mjs

 import { existsSync, readFileSync } from 'fs';
-import * as path from 'path';
 
 import {
   buildCSS,
@@ -18,7 +17,6 @@ import manifest from './scripts/gulp/manifest.js';
 import serveDev from './dev-server/serve-dev.js';
 import servePackage from './dev-server/serve-package.js';
 
-const STYLE_DIR = 'build/styles';
 const FONTS_DIR = 'build/fonts';
 
 gulp.task('build-js', () => buildJS('./rollup.config.mjs'));