error instead of warn for missing exports

[amk221]

Currently, this:

import { foo } from 'bar';

will cause Webpack to warn if foo does not exist, but this is easily missed. Meaning its possible to ship broken code.

This PR configures strictExportPresence to error instead of warn.

But this is actually deprecated in favour of exportsPresence, which I did try but it wasn't available on the current version.

Also note it will be true by default in v6 of webpack anyway.

Reading:

• https://stackoverflow.com/questions/40539768/webpack-throw-error-on-missing-member-import
• Why doesn't strictExportPresence default to true? webpack/webpack#14659

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Regarding tests, since I'm not familiar with the embroider repo I found it hard to know where they should go. I saw some that seemed like an appropriate place, but I'll need help tbh.