Interest in `ExtractStrict`?

[ekilah]

Any interest / reason to not want this type added? I can make a PR if so:

type ExtractStrict<T, U extends T> = T extends U ? T : never

context:

• Design Meeting Notes, 4/3/2019 microsoft/TypeScript#30738
• add stricter Omit helper type microsoft/TypeScript#30825
• Add OmitStrict type for when you don't want to be permissive. #31

Seems like the TS team's reasoning for making their official Omit non-strict included the fact that Extract was also non-strict, which is something I just ran into wanting. Just like OmitStrict, ExtractStrict will get the compiler to help out during refactoring things like this, or to prevent typos in the string keys:

export type TSomeColorOptions = ExtractStrict<TColor, 'blue' | 'whitee'> // should error for typo

[pelotom]

Sure, I'd take a PR for that. Also might as well add ExcludeStrict.