[Feature] Add ESLint plugin for js-toolkit best practices

[titouanmathis]

🔗 Linked issue

N/A

❓ Type of change

• 📖 Documentation (updates to the documentation, readme or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Adds a new @studiometa/eslint-plugin-js-toolkit package — an ESLint v9 / oxlint plugin that enforces best practices for projects using @studiometa/js-toolkit.

20 rules across 5 categories:

• Class structure: require-config, require-config-name-pascal-case, refs-camel-case, refs-plural-multiple, options-camel-case, refs-no-bracket-access, require-refs-declared-in-config
• Lifecycle: async-lifecycle-methods
• Event handlers: on-handler-naming, on-global-handler-prefix, no-event-listener-methods
• Forbidden patterns: no-deprecated-properties, no-dispatch-event, no-shadow-dom, no-create-app, no-deep-utils-import, no-shadow-dom
• Preferred patterns: no-redundant-with-mount-when-in-view, no-manual-intersection-observer, no-manual-mutation-observer, prefer-ref-over-query-selector

The plugin ships a configs.recommended flat config entry with sane defaults. The .oxlintrc.json at the root is updated to use the plugin locally.

📝 Checklist

• I have linked an issue or discussion.
• I have added tests (if possible).
• I have updated the documentation accordingly.
• I have updated the changelog.

[titouanmathis]

See #727