Mark Omit as deprecated

[Andyuu]

As explained in Type Fest and other discussions, Omit<> has some major limitations and is not recommended.

I found out about this only today, so I would like to see this marked as @deprecated here to guide others. Perhaps some additional explanation may be needed with the alternative recommendation of Type Fest Except<>.

[uhyo]

Thanks for the suggestion. I'm uncertain as to what to do here...

A half of me says we should just mark Omit as deprecated as suggested in this issue.

The other half says we could go further and improve the Omit definition.