Unresolved imports should error in fenced code blocks

[mbostock]

We translate static imports to dynamic imports, but this should error rather than resolve to undefined:

import {doesNotExist} from "./module.js";

[mbostock]

Currently the above is translated into (approximately):

const {doesNotExist} = await import("./_import/module.js");

We’ll need to do something like this instead:

const $module = await import("./_import/module.js");
if (!("doesNotExist" in $module)) throw new SyntaxError("The requested module './module.js' does not provide an export named 'doesNotExist'");
const {doesNotExist} = $module;

Where $module doesn’t collide with any referenced symbol.