meta: add .mailmap entry for new email for existing contributor

PR-URL: #39431
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

.mailmap

@@ -108,6 +108,7 @@ Danielle Adams <adamzdanielle@gmail.com> Danielle Adams <danielle.adams@heroku.c
 Danny Nemer <hi@dannynemer.com> <DannyNemer@users.noreply.github.com>
 Danny Nemer <hi@dannynemer.com> <hi@DannyNemer.com>
 Darshan Sen <raisinten@gmail.com>
+Darshan Sen <raisinten@gmail.com> <darshan.sen@postman.com>
 Dave Pacheco <dap@joyent.com> <dap@cs.brown.edu>
 David Cai <davidcai1993@yahoo.com> DavidCai <davidcai1993@yahoo.com>
 David Mark Clements <david.clements@nearform.com>