[WIP] chore(eslint): upgrade eslint to v9

[liady]

Temporary description for reviewers:

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.

WHY are these changes introduced?

Fixes #0000

WHAT is this pull request doing?

HOW to test your changes?

Post-merge steps

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	January 27, 202511:19 AM
metaobjects	✅ Successful (Logs)	Preview deployment	Inspect deployment	January 27, 202511:19 AM
custom-cart-method	✅ Successful (Logs)	Preview deployment	Inspect deployment	January 27, 202511:19 AM
classic-remix	✅ Successful (Logs)	Preview deployment	Inspect deployment	January 27, 202511:19 AM
third-party-queries-caching	✅ Successful (Logs)	Preview deployment	Inspect deployment	January 27, 202511:19 AM

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