tools: import only highlight.js core

Importing everything from highlight.js won't work in without-intl
builds. Import only core, as the code is already written to load
individual languages as needed.

Fixes: nodejs#41077

tools/doc/html.mjs

@@ -22,7 +22,7 @@
 import fs from 'fs';
 import path from 'path';
 
-import highlightJs from 'highlight.js';
+import highlightJs from 'highlight.js/lib/core';
 import raw from 'rehype-raw';
 import htmlStringify from 'rehype-stringify';
 import gfm from 'remark-gfm';