Eslint fixes and adding a prettier config for JS formatting

[douglowder]

Description of proposed changes

• Fix some existing eslint errors and warnings
• Allow higher versions of node to be used
• Add a config for the prettier JS formatter that is minimally invasive and compatible with the current .eslintrc being used

Testing

yarn run lint should return no errors.

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[jameshadfield]

Thanks for this work -- I've heard about prettify but never used it, so this PR have me a chance to play with this. I ended up rebasing this PR onto master as branch "prettify". All in all, it changes pretty much every file in the repo! I need to read a bit more about this and have a think about whether this is the direction we want to go, vs enforcing a subset of these style rules via eslint.