Add .FromNullable

[j0rn32]

#875

Adds .FromNullable function to simplify working with null values:

[ValueObject<int>] public readonly partial struct CompanyId;

...
var result = SomeFunction.GetSomeStuff();
// CompanyId? id = result.CompanyId is null ? null : CompanyId.From(result.CompanyId.Value);
CompanyId? id = CompanyId.FromNullable(result.CompanyId);