Add Prettier to codebase

[ahlec]

This PR is focused on adding a code formatter, bringing all of the existing code into compliance with the formatter, and then adding a validation workflow to ensure future code is all formatted correctly too.

Notes:

• I've added an empty "prettier": {} to the package.json file in order to support IDEs that are set up to only format if the repository is configured with Prettier
  • In VS Code (using the official Prettier plugin), this is the Prettier: Require Config setting
  • For these IDEs, the existence of Prettier in node_modules isn't enough; you need to have it explicitly "turned on," so I've turned it on without overriding any of the values
• I had Prettier skip over two directories for now:
  • .next is a local compile cache, so we get nothing from formatting it
  • @types is where we currently store our GraphQL codegen files. Codegen recommends against running the output through formatters in a second pass — if you're going to format it, it should be part of the codegen process itself, rather than something afterwards
  • This blocklist also already inherits from the default Prettier .prettierignore, meaning that it's also skipping node_modules/ and .git, etc.
• The PR validation workflow is a relatively simple/straightforward version right now. I've put it through a couple of tests on my fork to ensure it works:
  • Workflow passes (and caches correctly) when all files are in compliance: here
  • Workflow fails when one or more files isn't formatted: here
  • Workflow restores from an older cache when we don't have a direct cache hit: here