fix: add project-level .eslintrc.js to align lint setup

[laurelfulford]

Changes proposed in this Pull Request:

This PR adds a project-level .eslintrc.js, following the same format that newspack-plugin, newspack-blocks, and newspack-popups use. This repo was missing it, which meant npm run lint:js was falling back to @wordpress/scripts's default config instead of newspack-scripts.

This fixes an issue that came up in #320, where a trunk merge caused a peer-dep error in CI. Getting the repo matching other Newspack ones seemed like the safest fix.

How to test the changes in this Pull Request:

1. Run npm ci to install deps.
2. Run npm run build.
3. Run npm run lint:js
4. Confirm none of the above causes errors, and that you don't need to use --legacy-peer-deps to get the install to work.

[Copilot]

Pull request overview

This PR adds a project-level ESLint configuration so npm run lint:js uses the newspack-scripts ESLint rules (instead of falling back to @wordpress/scripts defaults), aligning this repo’s lint setup with other Newspack projects.

Changes:

• Add a root .eslintrc.js that extends newspack-scripts’ ESLint config.

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[laurelfulford]

Thanks @chickenn00dle! 🎉