Switch "DT" symbol to "TS" on npm

[karlhorky]

First of all, thanks for fetch-jsonp, useful library!

What?

Add a "types" field to package.json, like Jest and other package including their own types.

Why?

On the npm page for fetch-jsonp, there is a "DT" symbol, indicating that the library has 3rd-party DefinitelyTyped types:

The "DT" symbol links to @types/fetch-jsonp, which has a deprecation notice:

This is a stub types definition. fetch-jsonp provides its own type definitions, so you do not need this installed.

npm should report the correct types source for the library, with the blue "TS" symbol as can be seen on the npm page for jest:

The jest package includes its own types and the npm page shows the "TS" symbol, even though the package @types/jest also exists.