Incremental Typescript Compilation

[stramel]

With Typescript 3.4, an --incremental flag landed. This greatly speeds up builds. It would be great to be able to take advantage of this improvement. Hopefully, this can be an improvement on #6406, I know everyone is looking forward to TypeScript v3 😛

https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-4.html
https://devblogs.microsoft.com/typescript/announcing-typescript-3-4/

Here is their example of compile times with and without the flag:

Step	Compile Time
Compile without --incremental	47.54s
First compile with --incremental	52.77s
Subsequent compile with --incremental with API surface change	30.45s
Subsequent compile with --incremental without API surface change	11.49s

[ianschmitz]

This is already supported in fork-ts-checker-webpack-plugin which is what we use to do type checking. This improvement is included in the 3.0 alpha that we have out now. Feel free to try it out to see if you notice any improvements!