ambient typescript definitions moved to a separate package

[bjouhier]

The ambient definitions were bundled with streamline-runtime, which created problems with npm link because the definitions are copied in this case and the TS compiler does not like this.

I have solved the problem by moving the index.d.ts file out of streamline-runtime and into an new streamline-typings package. I'm working on a proper @types/streamline-runtime but this is not ready yet and streamline-typings can fill the gap.

See https://github.com/Sage/streamlinejs/wiki/TypeScript-support#enabling-streamline-in-a-typescript-project