Add mjs extension to eslint config

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The following discussion from !142846 (merged) should be addressed:

• @svedova started a discussion: (+5 comments)

  question This one throws an error on my IDE (https://github.com/import-js/eslint-plugin-import/blob/v2.29.1/docs/rules/extensions.md#importextensions). I think we're not watching .mjs for eslint. Is this a known/wanted behaviour? 🤔