Well scoped PRs that added interesting features

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I'd like to do a guided PR walkthrough video for PRs like:

• Unions - Union Types microsoft/TypeScript#824
• Async Functions - Async Functions for ES6 microsoft/TypeScript#3078
• Control Flow Analysis - Control flow based type analysis microsoft/TypeScript#8010
• String Literal Types - String literal types microsoft/TypeScript#5185
• Using JSDoc to extract types - JSDoc support microsoft/TypeScript#6024
• JS in TS - JavaScript LS scaffolding + JS module inference microsoft/TypeScript#5266
• Null types - Non-nullable types microsoft/TypeScript#7140
• Strict Functions - Strict function types microsoft/TypeScript#18654
• TSX - JSX and as operator microsoft/TypeScript#3564
• Optional Chaining - Add support for Optional Chaining microsoft/TypeScript#33294
• Unknown - New 'unknown' top type microsoft/TypeScript#24439
• Mapped Types - Mapped types microsoft/TypeScript#12114
• Rest Types - Rest type microsoft/TypeScript#13470
• Type Aliases - Type aliases microsoft/TypeScript#957
• Intersection Types - Intersection types microsoft/TypeScript#3622