USWDS-Next - CI: Create a GH action to test code formatting

[mejiaj]

Closes #33.

A new CI test that runs prettier on push.

Before	After

Major changes

• GitHub actions npm caching: For performance, I've added global npm caching to GitHub actions.
• Added prettier config: Added .prettierrc and .prettierignore as best practice.
• Added formatting tests: New tests for testing JS [npm runprettier:js] and MDX [npm run prettier:md] files. Formatting fixes included.

How to test

1. Run npm run prettier:js && npm run prettier:md.
2. There should be zero errors.
3. Modify a JS file to add an error like ${ this.href }.
4. You should see a code style issue.
5. Modify an MDX file to add an error like { Readme }.
6. You should see a code style issue.
7. Run npm run prettier:js:fix && npm run prettier:md:fix to fix issues.
8. Styles should match after re-running npm run prettier:js:fix && npm run prettier:md:fix.

[heymatthenry]

Thanks for doing this! Automate all the things 🤖

[heymatthenry]

thought: At some point we may need to move tasks around between actions to cut down on the number of times we're running npm ci in CI

[mejiaj]

That's a great thought.

The setup-node action supports caching. Added in ef95346.

You should now see a caches link in actions.

Not seeing any major differences so far.

Before	After

https://github.com/uswds/uswds-next/actions/caches

---

Tip

Monorepo dependencies can be cached too. Example below.

Monorepo dependency cache

Details

steps:
- uses: actions/setup-node@v4
  with:
    node-version: '18.x'
    cache: 'npm'
    cache-dependency-path: '**/package-lock.json'
- run: npm ci
- run: npm test

Source: https://github.com/actions/setup-node?tab=readme-ov-file#caching-global-packages-data

[heymatthenry]

suggestion (non-blocking): Doesn't have to be in this PR but it might be a good idea to add a minimal prettier config even if we're just using defaults. Some editors need a config in order to run prettier, and it's just helpful to have at the project root to signal intent.

[mejiaj]

Agreed with clear intent.

Added .prettierignore and .prettierrc in 6342c0d.

[mejiaj]

@heymatthenry thanks for the suggestions.

Changes were minor, so I've added them and updated the PR description to include the new info.

[heymatthenry]

Thanks for this! Merging it in so I get all the 🤖 goodness while I dig into package configs 😎