Nullable<T> vs Maybe<T>

[xavierjohn]

Is there a reason to use Maybe anymore compared to Nullable?
We could add a ToResult extension to Nullable.

[xavierjohn]

I decided to ditch Maybe since it did not feel natural in VS 2022. Maybe was a good option in the past.
May be we can conditionally add something like https://github.com/xavierjohn/FunctionalDDD/blob/main/RailwayOrientedProgramming/src/Result/Extensions/ToResult.cs where nullability is supported.

[vkhorikov]

Nullable only allows structs as type arguments.

[xavierjohn]

This PR will add ToResult for Null & Nullability.
#494

Some of the Target Framework seems redundant so I have removed it. .NET Framework users can use the library that targets .NET standard 2.0.