Update @types/showdown requirement from 1.9.0 to 1.9.1

Updates the requirements on [@types/showdown](https://github.com/DefinitelyTyped/DefinitelyTyped) to permit the latest version.
- [Release notes](https://github.com/DefinitelyTyped/DefinitelyTyped/releases)
- [Commits](https://github.com/DefinitelyTyped/DefinitelyTyped/commits

)

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