Migrate to ESLint 9 with flat config and fix monorepo linting

[better-salmon]

What is it?

• Bug

Description

This PR fixes a significant issue where ESLint wasn't functioning properly (silently crashing) across most of the monorepo files. The changes include:

1. Migration from legacy ESLint config to ESLint 9 with flat configuration
2. Improved lint script execution by properly separating local ESLint configs from the root config (ensuring they no longer overlap — errors in IDE and from the script will be the same)
3. Removal of unnecessary eslint-disable comments throughout the codebase
4. Fixed all new errors introduced by the newer config that were previously missed while ESLint wasn't working
5. Use eslint.config.js by default in starters (Fix: [✨] Update your template to include eslint.config.js by default #6115)

These improvements ensure proper linting across the entire codebase and enable better code quality enforcement going forward.

Checklist

• My code follows the developer guidelines of this project
• I performed a self-review of my own code
• I added a changeset with pnpm change
• I made corresponding changes to the Qwik docs
• I added new tests to cover the fix / functionality

[changeset-bot]

🦋 Changeset detected

Latest commit: 3023245

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 4 packages

Name	Type
create-qwik	Patch
@builder.io/qwik-city	Patch
@builder.io/qwik	Patch
eslint-plugin-qwik	Patch

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npm i https://pkg.pr.new/@builder.io/qwik@7418

npm i https://pkg.pr.new/@builder.io/qwik-city@7418

npm i https://pkg.pr.new/eslint-plugin-qwik@7418

npm i https://pkg.pr.new/create-qwik@7418

commit: 3023245

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[wmertens]

this is great! but some small nitpicks

[wmertens]

better to just remove it, no?

[better-salmon]

I've noticed that the no-unused-vars rule is disabled in several places, so I've disabled it here as well for consistency. In my opinion, it's better to maintain uniform rules across all packages in the monorepo. However, enabling this rule everywhere right now would introduce many errors, so perhaps it's best to address that separately in another PR.

[wmertens]

I meant } catch { without the (e)

[better-salmon]

@wmertens, I also noticed that your monorepo setup isn't optimally configured. I could set up turborepo and shared ESLint and TypeScript configurations. However, since this would be a sizable PR, I wanted to confirm with you before starting any work 🙂

[maiieul]

@better-salmon a decision was made in the past to not use Nx to avoid unnecessary overhead. Maybe it would work well, but maybe it could cause bugs that are hard to pinpoint. Same with Turborepo I believe. Also such tooling might turn off potential contributions from devs that don't know the tool in question.

TLDR: I think that if you rebase we can merge your PR :)