Add Linter (code formatter)

- https://prettier.io/
- https://standardjs.com/

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- https://github.com/prettier/prettier#how-does-it-compare-to-eslint-or-tslint-stylelint
- https://github.com/sheerun/prettier-standard
- https://www.reddit.com/r/javascript/comments/6g8adf/prettier_has_surpassed_standard_as_mostdownloaded/
- https://testingjavascript.com/courses/static-analysis-testing-javascript-applications