Fix the Observable Inputs ESM distribution

[mbostock]

It’s missing two things:

• The stylesheet isn’t included in the npm distribution so we have to load it from GitHub.
• The stylesheet and the ESM source use __ns__ as the namespace rather than oi-3a86ea.

This is really a bug in the Inputs repo, but I’m filing it here because this is the first time we’re consuming it as an ES module.

[mbostock]

I think this can be done without really being user-facing so lowering priority.

[mbostock]

Fixed in #469.