mdoc.js: support ES modules and SmallModulesFor

[armanbilge]

Scala.js 1.10.0 added ModuleSplitStyle.SmallModulesFor to enable fast, iterative development of Scala.js apps.

The typical usage pattern is to list the application’s packages as argument. This way, often-changing classes receive independent, small modules, while the stable classes coming from libraries are bundled together as much as possible.

https://www.scala-js.org/news/2022/04/04/announcing-scalajs-1.10.0/#new-module-split-style-smallmodulesforpackages

IIUC all mdocs are compiled under the mdoc package so I expect that would be the argument to use for this setting. Then, mdoc --watch can provide a supercharged dev cycle.

[armanbilge]

Btw, I don't think this should be too hard to support. I think the key change here is adding support for ESModules, which means:

1. supporting multiple JS file output from the linker
2. Using in the browser with <script type="module" src="..."></script>

After enabling ES modules for (the non-mdoc part of) my project, the only change I had to make was (2).