lib,src,doc: add --heapsnapshot-signal CLI flag

This flag allows heap snapshots to be captured without
modifying application code.

PR-URL: https://github.com/nodejs/node/pull/27133


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>