Feature Request: Support an ES Modules Dist Build

[jimniels]

Today, if you want to user mercury in the browser, you can pull it in via a <script> tag from a CDN like unpkg as an iife.

<script src="https://unpkg.com/@postlight/mercury-parser@2.2.0/dist/mercury.web.js"></script>

Would be nice if there was an ES modules build, so then you could import it directly:

import Mercury from "https://unpkg.com/@postlight/mercury-parser@2.3.0/dist/mercury.esm.js"

It seems like this would be pretty straightforward. I'm not entirely sure how the build is set up, but mimicking what I saw with the build:web scripts, I added one for ES Modules. Tested locally and it produced a build file (dist/mercury.esm.js) which I could import myself in a local project.

Note: I had to use terser instead of uglify for ES modules. See this issue.

[jimniels]

CI is failing, but doesn't appear to be my fault...