src: rename `On*` -> `Emit*` for stream callbacks

This should make these function calls a lot more intuitive for people
who are more accustomed to Node’s EventEmitter API.

PR-URL: https://github.com/nodejs/node/pull/17701


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>