Add TryDehumanizeTo with nullable

[adamhathcock]

Adding a new method to feel slightly cleaner when testing if an enum can be dehumanized.

Addresses https://github.com/Humanizr/Humanizer/issues/769

[adamhathcock]

I believe the failures are because I changed the public API. Will wait to see what I should to fix, if approved.

[SimonCropp]

@adamhathcock LGTM. can you rebase and approve the api change

[SimonCropp]

I recently added the below, which should meet this requirement

public static TTargetEnum? DehumanizeTo<TTargetEnum>(this string input, OnNoMatch onNoMatch = OnNoMatch.ThrowsException)

[adamhathcock]

Thank you @SimonCropp