Eslint all files with prettier

[dgreif]

ESLint was previously set up to do the default file glob, which only finds .js files. With all the awesome TypeScript conversion @colebemis has been doing, lint has been covering less and less files. This PR does the following:

• adds coverage back to all files (.js, .ts, .tsx)
• adds the prettier plugin into the eslint config (it was a dependency, but wasn't being used)
• fixes all the related lint issues
• adds yarn lint-fix which can be used to run prettier if your IDE isn't already configured to do so
• updates all the eslint/prettier related dependencies.

Note: I split the base eslint fixes and the prettier fixes into two separate commits. The base fixes probably deserve the closer look of the two.

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[colebemis]

Thank you for doing this, @dgreif! ❤️

These changes look great. However, I tried running yarn lint locally on this branch and my computer fans started running on high and the script never finished. When I change the script to eslint 'src/**/*.{js,ts,tsx}' instead of eslint '**/*.{js,ts,tsx}' the script runs as expected. It seems like the ignorePatterns field might not be working as expect (at least on my machine).

[dgreif]

Hmm, there must be another build output dir that I missed. @colebemis can you think of another spot that .js files are generated? I'd prefer not to limit it to src/... because there are lots of other one-off js files that wouldn't get picked up.

[colebemis]

Thanks for taking the initiative to fix this, @dgreif 🚀