chore(eslint): upgrade eslint to v9

[liady]

WHY are these changes introduced?

Fixes #191

WHAT is this pull request doing?

This PR:

• upgrades eslint to v9 and its corresponding plugins, packages and configs
• removes individual eslint from all of our packages, except templates/skeleton
• unifies our basic lint configuation to be in the root of the monorepo (it was empty until now)
• fixes our lint-staged script to run on the changed files
• removes deprecated dependencies of remix-run lint plugin and hydrogen lint plugin (not maintained)

The relevant files in the PR are:

• eslint.config.js - this is the base eslint config for linting our codebase
• templates/skeleton/eslint.config.js - this is the skeleton-specific configuration, and it can be removed by the skeleton users if they want.
• the rest are (stylistic only) lint fixes

I tried to find the best subset of rules so that we will be able to have even minimal linting over our codebase, but at the same time not to flood many warnings over packages that weren't linted until now. If in the future we will want to add/enable a new rule, we can do it and fix its errors.

HOW to test your changes?

1. npm i && npm run lint from the root folder
2. npm i && npm run lint from the templates/skeleton folder

Checklist

• I've read the Contributing Guidelines
• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've added a changeset if this PR contains user-facing or noteworthy changes
• I've added tests to cover my changes
• I've added or updated the documentation

[shopify]

Oxygen deployed a preview of your chore/eslint-upgrade branch. Details:

Storefront	Status	Preview link	Deployment details	Last update (UTC)
Skeleton (skeleton.hydrogen.shop)	✅ Successful (Logs)	Preview deployment	Inspect deployment	February 3, 2025 6:33 PM
metaobjects	✅ Successful (Logs)	Preview deployment	Inspect deployment	February 3, 2025 6:33 PM
custom-cart-method	✅ Successful (Logs)	Preview deployment	Inspect deployment	February 3, 2025 6:33 PM
classic-remix	✅ Successful (Logs)	Preview deployment	Inspect deployment	February 3, 2025 6:33 PM
third-party-queries-caching	✅ Successful (Logs)	Preview deployment	Inspect deployment	February 3, 2025 6:33 PM

Learn more about Hydrogen's GitHub integration.

[github-actions]

We detected some changes in packages/*/package.json or packages/*/src, and there are no updates in the .changeset.
If the changes are user-facing and should cause a version bump, run npm run changeset add to track your changes and include them in the next release CHANGELOG.
If you are making simple updates to examples or documentation, you do not need to add a changeset.

[wizardlyhel]

This is generated automatically. Deleting it here doesn't do anything since a npm run dev would bring this back. We should ignore eslint on this file.