tools,doc: fix misrendering of consecutive JS blocks

Our markdown-to-html tool was assuming that any consecutive JS blocks
were ESM vs CJS alternatives, but that is not always the case, resulting
in both a confusing interface and invalid HTML.

PR-URL: https://github.com/nodejs/node/pull/40146


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>