tools: Include links to source code in documentation

Parse source code using acorn; extracting exports.  When producing
documentation, match exports to headers.  When a match is found, add a [src]
link.

This first commit handles simple exported classes and functions, and does so
without requiring any changes to the source code or markdown.  Subsequent
commits will attempt to match more headers, and some of these changes are
likely to require changes to the source code and/or markdown.

PR-URL: https://github.com/nodejs/node/pull/22405


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>