prettierrc: reduce merge conflicts, improve git blame w/trailingComma: 'all'

[devinrhode2]

The least subjective recommendation I have for reducing merge conflicts, and improving git blame, would be to set prettier trailingComma option to 'all', see: https://prettier.io/blog/2020/03/21/2.0.0.html#change-default-value-for-trailingcomma-to-es5-6963httpsgithubcomprettierprettierpull6963-by-fiskerhttpsgithubcomfisker

[devinrhode2]

As discussions about mass formatting change are underway (#932) I think it's a timely discussion. We ideally should not do repeated mass formatting changes, ideally we just do one and hopefully can avoid doing another for a very long time

Cc @orenelbaum