Add index.d.ts typings file

[NotWoods]

My continued efforts to fully 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.

Fixes #137

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Since the plugin accepts the same options as TypeScript's compiler options, I initially experimented with extending the exported CompilerOptions type. Unfortunately that interface doesn't match up with the JSON schema, and uses enums for certain values like target. I ended up just using an index type instead, although the types are slightly weaker. TypeScript just uses any for its JSON parsing function so this should be fine.

[shellscape]

@NotWoods if you're up for fixing the conflict, I'll merge this immediately.

[NotWoods]

Updated!