Fix master: Pin TypeScript dependency

[NickColley]

TypeScript does not follow Semantic Versioning.

This means that the caret (^) syntax used by npm installed a breaking version, as it expects minor and patch releases to be backwards compatible.

This unfortunately means we need to pin the dependency to ensure our code does not break.

export * is Always Retained
In previous TypeScript versions, declarations like export * from "foo" would be dropped in our JavaScript output if foo didn’t export any values. This sort of emit is problematic because it’s type-directed and can’t be emulated by Babel. TypeScript 3.9 will always emit these export * declarations. In practice, we don’t expect this to break much existing code, but bundlers may have a harder time tree-shaking the code.

https://devblogs.microsoft.com/typescript/announcing-typescript-3-9/#breaking-changes