Convert to ESLint Shareable Config, eslint-config-pa11y

[danyalaytekin]

This PR updates the project to become an ESLint Shareable Config and adds a test in CI to verify that it can be installed and used. These insist on the prefix eslint.

Changes

• Rename package.name to eslint-config-pa11y from pa11y-lint-config
• Test installation and usage of the config into a temporary project in GitHub Actions, referencing it by the name pa11y
  • this can be parameterised in future to cover specific versions of ESLint

[hollsk]

🎉

[danyalaytekin]

Thanks for the review @hollsk. I'll fix the issue that has arisen in main. I think it's because ESLint released version 9 since the tests ran previously, and the new test script introduced here just installs the latest version of ESLint. I suppose in a way it's good that it caught this!

• Fix client project test #12