feat: upgrade eslint to v9

[benmccann]

We tried before and had to rollback because typescript-eslint v7 was not compatible. Try again this time with a pre-release version, which seems to be quite stable. Otherwise we'd have to make all users wait another 2-4 months

Ref typescript-eslint/typescript-eslint#9141

[changeset-bot]

🦋 Changeset detected

Latest commit: f0dcf0d

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

This PR includes changesets to release 1 package

Name	Type
create-svelte	Minor

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[hyunbinseo]

I have read the typescript-eslint issue and understand that the beta release is 'behaviorally stable'.

• Try out v8 beta on various influential community repos typescript-eslint/typescript-eslint#9141

but it still feels weird to have an alpha dependency in the official template.

Would pnpm update upgrade 8.0.0-alpha.20 to 8.0.0 once it is officially released?

[benmccann]

Yeah, it is a bit weird. But it's much easier for users to just upgrade that one package than to do the whole migration from eslint v8

[hyunbinseo]

Migration Guide

For Prettier users, you can reference this diff.

1. ESLint v9's default config is flat config, and this is used. Release Note
2. (Optional) typescript-eslint package that uses the flat config has a different package name. Release Note

- "@typescript-eslint/eslint-plugin": "^7.10.0",
- "@typescript-eslint/parser": "^7.10.0",
+ "typescript-eslint": "8.0.0-alpha.20",

# JavaScript
pnpm i eslint@9 globals@latest -D

# TypeScript
pnpm i eslint@9 globals@latest typescript-eslint@rc-v8 -D
pnpm rm @typescript-eslint/eslint-plugin @typescript-eslint/parser

3. Legacy ESLint config should be removed, and a new flat config should be added.

+ eslint.config.js
- .eslintrc.cjs
- .eslintignore # The ignore file should be removed!

The content of the config files differs based on (TypeScript, Prettier) installation.

Reference the Files changed tab in this PR.

[niemyjski]

I upgraded my ts app following these changes and now I'm seeing lots of errors around generics

7:29 error 'T' is not defined no-undef
8:16 error 'T' is not defined no-undef

<script lang="ts" generics="T">
  import type { Snippet } from 'svelte';
  import type { HTMLAttributes } from 'svelte/elements';
  import { cn } from '$lib/utils';

  type Props = HTMLAttributes<Element> & {
      children?: Snippet<[T]>;
      items: T[];
  };

8:18 error '$$Generic' is not defined no-undef

<script lang="ts">
 import IconViewColumn from '~icons/mdi/view-column';
 import { Button } from '$comp/ui/button';
 import * as DropdownMenu from '$comp/ui/dropdown-menu';

 import type { Table } from '@tanstack/svelte-table';

 type TData = $$Generic;
 export let table: Table<TData>;
</script>