Add support for import assertion resolution

[Westbrook]

Allow code like:

import styles from 'x/styles.css' with { type: 'css' };

to be processed down to

import styles from './node_modules/x/styles.css' assert { type: 'css' };

There is zero support or with { ... } as yet, so it seemed like processing to something with at least a little support was the right call for right now.

Possibly worth do a test with assert for the currently available support, so let me know if you'd like that added as well.

Looking to leverage this in tachometer to measure the perf benefits of switching from component.css.js file to component.css files in packages for https://opensource.adobe.com/spectrum-web-components/

[Westbrook]

Also, I ended up doing a bunch of updating before actually updating the API consumption which might not have been needed if I started with the code changes. Let me know if it's worth trying to reduce the amount of changes taken on in this PR.

[Westbrook]

Oh, hey, any chance of getting this added to the library?

[Westbrook]

When this lands/releases, tachometer is on "koa-node-resolve": "^1.0.0-pre.8", so in theory a 1.0 or 1.0.0-pre.9 would auto resolve in my projects? If not, we may need a release of both...