Emit the deprecation warning in the `kDefaultUnhandledRejections`
case to reduce the number of branches on unhandled rejection mode -
there is now only one switch case on it.
Also rename `emitWarning()` to `emitUnhandledRejectionWarning()`
to avoid ambiguity with `process.emitWarning()`
PR-URL: https://github.com/nodejs/node/pull/28258
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>