Two flavors of literal types

Widening and non-widening https://github.com/Microsoft/TypeScript/pull/11126

Need to add docs here https://github.com/basarat/typescript-book/blob/master/docs/types/literal-types.md :rose:

Open docs/types/literal-types.md and read the linked TypeScript PR #11126 to understand the widening and non-widening literal type changes. Update that documentation page to cover both flavors, and consider the work done when the page explains the distinction accurately and clearly.

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