Eslint rule updates

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• Installs https://github.com/sindresorhus/eslint-plugin-unicorn
• Installs https://github.com/SonarSource/eslint-plugin-sonarjs
• Installs https://github.com/import-js/eslint-plugin-import (Only for a single rule, to enforce file extensions for JS imports.)

The general process I have tried to follow for the recent eslint rule updates, including this one, is to try to enable everything, and then disable things which don't match the existing style for the most part.

Resolves #615.

[github-actions]

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[toasted-nutbread]

I've left comments on a few of the noteworthy things I ran into while making this update. @djahandarie feel free to read them and just close them, as I don't have actions for them aside from just noting them.

Beyond that, if we run into any future issues with these rules causing trouble, I'm not opposed to disabling the problematic ones.