ci: add overall prettier check to ci

[MikeMcC399]

Situation

• Previous PRs have made incremental changes to the scope of Prettier formatting and checks

• Running npx prettier . --check now returns success

• Scripts to run Prettier across the whole repo and in CI are missing

• The use of Prettier is not documented

Change

• Add script
  • format:all to fix all required formatting with Prettier
  • format:all:check to check all required formatting with Prettier
• Add instructions to CONTRIBUTING
• Add format:all:check to .github/workflows/main.yml

Verification

On Ubuntu 24.04.2 LTS, Node.js 22.14.0 LTS

npm ci
npm run format:all:check

and confirm the response is:

Checking formatting...
All matched files use Prettier code style!

[cypress-app-bot]

• Create a Draft Pull Request if your PR is not ready for review. Mark the PR as Ready for Review when you're ready for a Cypress team member to review the PR.

[github-actions]

🎉 This PR is included in version 6.8.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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