Add .d.ts typings file

[NotWoods]

Adds a .d.ts TypeScript declaration file to support typechecking a Rollup config with TypeScript.

TypeScript allows for checking normal Javascript files by adding a // @ts-check comment at the top of the file. By providing typings users can check that they aren't passing misspelled or incorrect parameters, and see a hint about what each option does in their editor.

[lukastaegert]

Sorry for letting this wait for so long. I would actually like to merge this one and I see you created some kind of test. Would it be possible to "run" this test as part of the test script? I assume you will want to add TypeScript as a dev dependency, which is definitely not a problem.

[NotWoods]

Yup, the type checking tests can be run automatically. I've added TypeScript and some minimal configuration to run the check.

[lukastaegert]

Looks good, awesome!

[nicolashenry]

@lukastaegert The typing file does not exists in the package available on npm registry.
I think this part is missing in package.json file :

"files": [
    "dist/*.js",
    "dist/*.d.ts"
  ],

And maybe also :

 "typings": "index.d.ts",

[NotWoods]

Yeah, I didn't notice the files field. Adding "index.d.ts" to files will do the trick, the typings/types field is unnecessary because TypeScript will automatically find files named index.d.ts in the package root.