Add ES module distribution

[itsMapleLeaf]

For use with modern bundling tools like Vite and Snowpack. At the moment, Vite tries to optimize the library as a CommonJS module with a single default export, then errors with this at runtime:

Uncaught SyntaxError: The requested module '/@modules/micro-observables.js' does not provide an export named 'observable'

Adding an ES module distribution would fix this. I'm also open to adding this myself

[itsMapleLeaf]

Ah, just noticed this is already in place in the 1.5.0 RC versions, very nice. Closing 😄