ESM module & TS typings

[bigtimebuddy]

Because resource-loader does not export an ES module build, it requires users that use Rollup + pixi.js to define named exports. For instance:

./rollup.config.js

import commonjs from 'rollup-plugins-commonjs';
export default {
  //...
  plugins: [ 
    commonjs({
      namedExports: { "resource-loader": [ "Resource" ] }
    })
  ]
}

It is creating a minor gotcha for users of Rollup and could be alleviated if an ES bundle was created and defined in the package.json module field.

In addition, TypeScript typings would also help improve developer experience.

[englercj]

TypeScript typings are generated here: https://github.com/englercj/resource-loader/tree/master/typings

I'm happy to make any changes that make things easier, but I'm not 100% sure what needs to change. Open to any PRs.