Support TypeScript declarationMap

[n4bb12]

When generating TS types with the declarationMap option, additional .d.ts.map files are generated.
https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-9.html#new---declarationmap

Code editors can then take you to the .ts source instead of .d.ts.
This makes dependencies navigable just like regular source code in your project.

You can see this in action e.g. with the appllo projects that let you jump to the implementation.
https://github.com/apollographql/apollo-server/blob/master/tsconfig.base.json#L10

If you are open to supporting this option, I would attempt a PR.

[swernerx]

Sounds good. Please try out to make a PR.