Exclude mjs extension from file-loader

[jgoux]

I excluded .mjs files from file-loader.

This should fix #5234

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[Timer]

Hmm, we need to think about this more because .mjs files currently aren't being processed via the same pipeline as .js, which means they're not being transformed via Babel, etc.

Is there a way to tell webpack to pretend .mjs files have the .js extension earlier up in the chain?

[Timer]

A couple things we need to be careful of:

1. mjs can not be permitted to work under src/. This import should fail and give you the URL.
2. mjs must be transformed via Babel.

Let's make sure we add tests for this behavior.