Add full support for mvnpm Maven artifacts

It sounds interesting to have proper support for packages coming from https://www.mvnpm.org.

You can install them just fine right now. But they are currently identified as regular jars, so they don't exactly have the same behavior as what one would be expected from what are essentially webjars (require only admin right to install, and tooling to produce proper URL to their included files).

So that means:

• adding a mvnpm extension type (identified using "org.mvnpm:" extension id prefix)
• add support for them in the "webjars" API (or introduce a "mvnpm" API)
• get the importmap from the importmap.js file