[Security] Bump y18n from 4.0.0 to 4.0.1 in /Rnwood.Smtp4dev/ClientApp

Bumps [y18n](https://github.com/yargs/y18n) from 4.0.0 to 4.0.1. **This update includes a security fix.**
- [Release notes](https://github.com/yargs/y18n/releases)
- [Changelog](https://github.com/yargs/y18n/blob/master/CHANGELOG.md)
- [Commits](https://github.com/yargs/y18n/commits)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>