Some classes does not implement async interface members

[attolee]

Enable USN_USE_ASYNC_METHODS, found that the following classes do not implement their interface,

1. AnonymousModalLifecycleEvent
2. AnonymousModalLifecycleEvent
3. AnonymousSheetLifecycleEvent

[Haruma-K]

@attolee
Indeed. I will fix it.
Thanks for the info!

[Haruma-K]

@attolee
I fixed it and released as 1.5.1!

https://github.com/Haruma-K/UnityScreenNavigator/releases/tag/1.5.1

PR is below.

#20

Thanks again for your information!