tools: add direct anchors for error codes

This adds direct anchors for the error codes in errors.html.
For example, previously the anchor for ERR_ASSERTION
is #errors_err_assertion, now there is also #ERR_ASSERTION.

PR-URL: https://github.com/nodejs/node/pull/16779


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: James M Snell <jasnell@gmail.com>