Add tsconfig.json to core-data package

[adamziel]

What?

This PR adds tsconfig.json to the @wordpress/core-data. It's a logical next step after adding all the useful TypeScript types.

Testing Instructions

Confirm the checks are green

cc @gziolo @dmsnell @sarayourfriend

[gziolo]

I see that TypeScript validation passes on CI. Is there anything left to expose types for the package on npm?

[adamziel]

I see that TypeScript validation passes on CI. Is there anything left to expose types for the package on npm?

I think so, yes 🎉 This PR gets all the remaining types to behave.

[gziolo]

It’s your call then whether to expose types in this PR or separately 👍

When exposing types we only need to remember to include a breaking change entry in the changelog. We definitely will have different types than those that live now on DefinitelyTyped.