Check formatting as a GH action

[chalin]

• This is a step in the direction of making the build process more contributor friendly.
• Allows build previews (and deploys) to succeed even if there are mis-formatted files.
• Makes format checking a GH action, making it clear (when the action fails) what the cause of the failure is (rather than having to dig into the deploy logs).
• Related: Create bot action to run formatter and link checker #2388

[chalin]

Successfully tested with misformatted file:

• Check formatting fails
• Deploy preview succeeds