Add a test to validate release assets

[mikachan]

Based on this PR that adds an AGENTS.md file: #53

This adds an npm test command that validates the output of npm run api and npm run previews before committing a new release. Tests use Node's built-in node:test runner and check that the generated JSON files are valid, structurally correct, and have corresponding SVG preview files on disk.

Also fixes a pre-existing ESLint misconfiguration that caused all JS files to fail npm run lint:js with a Babel error.

This makes it easier for both humans and agents to test if the release assets are correct.

To test, run: npm test and the new test should pass.

[scruffian]

All the tests pass. I get this error when running the linter:

wp-scripts lint-js

/Users/ben/Sites/google-fonts-to-wordpress-collection/tests/validate-release.test.js
132:9 error Replace ⏎↹↹slug,⏎↹}·=· with ·slug·}·=⏎↹↹ prettier/prettier
144:9 error Replace ⏎↹↹slug,⏎↹↹fontFace,⏎↹}·=· with ·slug,·fontFace·}·=⏎↹↹ prettier/prettier

✖ 2 problems (2 errors, 0 warnings)
2 errors and 0 warnings potentially fixable with the --fix option.

[mikachan]

Thanks!

I get this error when running the linter:

Looks like I had to update my linter locally. This should be fixed now.

[scruffian]

Thanks!