`bob build --incremental` for faster builds

[enisdenjo]

Currently bob deletes the build folder on every build. This is important to have a 100% working clean build. But doing a lot of build during development can slow you down significantly.

Most of the build time is consumed by TS. Since deleting the folder removes tsbuildinfo files too, TS cannot perform a faster optimised/incremental build.

Add --incremental build option to keep the build folder and leverage tsbuildinfo for faster TS builds.