Enable ".Which" extension for value-containing assertions

[sparerd]

This PR enables the FluentAsserts .Which sub-assertion helper when asserting on a value-containing state of a monad (BeSome, BeLeft, etc).

One odd thing that I found when implementing this is that LanguageExtTryAssertions.BeFail returns an Exception type when converted to an Either in order to grab the failure message. However, LanguageExtTryAsyncAssertions.BeFail returns an Error type. I've left this difference as is, but it may be confusing for an end user. I'm not exactly sure why LanguageExt is returning a different type in this case. We can either leave it, make both use Exception, or make both use Error. Not sure what the best choice is, so it is up to you.

Resolves #2

[jmojiwat]

Thank you so much for this, @sparerd. This is really useful.

[jmojiwat]

@sparerd thanks for filling in the tests for LanguageExtMonoidValidationAssertionsTest too.

I'm not sure why the return types are different either. I lean towards leaving it as it is, and just sync with LanguageExt if it decides to change its return type in the future.

Maybe we can revisit this if end users keep tripping up here.