Subtraction Types

Using a clever combination of `Pick` to create an `Omit`: (Source: https://github.com/Microsoft/TypeScript/issues/19569)

```
export type Diff = ({[P in T]: P} &
{[P in U]: never} & {[x: string]: never})[T]
export type Omit = Pick>
```

Better with conditional types : https://github.com/Microsoft/TypeScript/issues/19569#issuecomment-359471618

```
export type Diff = T extends U ? never : T; // Remove types from T that are assignable to U
export type Omit = Pick>
```

Review the TypeScript book’s existing coverage of Pick, Omit, and conditional types, then compare it with the two Diff/Omit examples and the linked TypeScript discussion. Done should be a clearly scoped decision about whether and where this material belongs, with the relevant explanation and examples updated if accepted.

Written by the indexing model from the issue text.