Investigate if new feature of typescript 3.3 can be used and help performance

[PopGoesTheWza]

Check out https://github.com/Microsoft/TypeScript-Handbook/blob/master/pages/release%20notes/TypeScript%203.3.md#incremental-file-watching-for-composite-projects-in---build---watch

[yorrd]

without actually having worked on it, from doing preliminary research it seems like we need to wait for this

microsoft/TypeScript#29978

[yorrd]

also heads up: there is a build cache that is persisted to .meteor/local/... atm from the old barbatus:typescript. I assume we can remove that one when we switch to the official incremental flag and slim down a bit on the size of the code of this project.

[perbergland]

TS 3.6 finally has an official API for incremental compilation

https://devblogs.microsoft.com/typescript/announcing-typescript-3-6/#apis-to-support-build-and-incremental