Provide an ES module build

[matthewp]

We could keep the global build as attr.js and add an attr.mjs as an ES module build... or just get rid of the global build. Thoughts @bedeoverend ?

[matthewp]

cc @FredKSchott this is one I might use pika/pack. I have a couple of needs here:

• An ES module build (the source will be changed to be ES web compatible).
• A UMD bundle that adds a global.

Not asking you to do anything, just letting you know I might try the tool on this :)