Add esm build and package exports

[jacob-alford]

We noticed that remote-data-ts only exposed a common-js version which was problematic for a few reasons, notably: it prevented tree-shaking for fp-ts. This PR adds a tsconfig.build.esm.json with module: ES2015 and target: ES6 and builds it to an _esm directory. Additionally it adds an exports field to the package json which points typescript, node, bundlers, and browsers to the right places.