feat: config-helpers package

[nzakas]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request?

Created a new config-helpers package

What changes did you make? (Give an overview)

• Added packages/config-helpers directory.

• .github/ISSUE_TEMPLATE/bug-report.yml and .github/ISSUE_TEMPLATE/change.yml: Added @eslint/config-helpers label to the issue templates. [1] [2]

• .github/workflows/release-please.yml: Added steps to publish the @eslint/config-helpers package to npm and JSR, and to post release announcements.

• .release-please-manifest.json: Added @eslint/config-helpers to the release manifest.

• README.md: Updated the list of packages to include @eslint/config-helpers.

Related Issues

Refs eslint/eslint#19116

Is there anything you'd like reviewers to focus on?

1. I purposely kept the README very light as I think the majority of the docs should be on the website.
2. I deviated from the RFC with automatic naming of configs as I found the RFC-described way difficult to debug. Instead, user configs now are UserConfig[index] and extended configs are now ExtendedConfig[index]. Using the bracket notation allows us to more clearly indicate the presence of a multidimensional array.
3. Also a deviation from the RFC is that I kept extends as an array of arrays at maximum instead of infinite nesting. I'd like to see how far we get with this.
4. I'm not sure if I should also update the eslint/migrate-config package in this PR or a separate one. I opted for a separate one for now, but let me know if you think differently.

[mdjermanovic]

CI Test Types fails with rollup: not found when building packages/compat. I'm not sure how the same steps in the same environment pass in the ubuntu-latest + Node.js 22.x job but fail in this one.

[mdjermanovic]

Ah, actually steps are not the same. Test Types runs npm install in package directories, but that somehow triggers the root prepare script. Maybe we could just do npm install in the root instead of in package directories.

rewrite/.github/workflows/ci.yml

Lines 57 to 80 in a57dd45

	test_types:
	name: Test Types
	runs-on: ubuntu-latest
	steps:
	- uses: actions/checkout@v4
	- name: Setup Node.js
	uses: actions/setup-node@v4
	with:
	node-version: "lts/*"
	- name: npm install and test types (core)
	working-directory: packages/core
	run: |
	npm install
	npm run build
	npm run test:types
	- name: npm install and test types (plugin-kit)
	working-directory: packages/plugin-kit
	run: |
	npm install
	npm run build
	npm run test:types

[mdjermanovic]

3. Also a deviation from the RFC is that I kept extends as an array of arrays at maximum instead of infinite nesting. I'd like to see how far we get with this.

Doesn't the current implementation allow infinite nesting?

rewrite/packages/config-helpers/src/define-config.js

Lines 385 to 387 in e72cdf5

	for (const { indexPath, value: extendsElement } of flatTraverse(
	objectExtends,
	)) {

For example, defineConfig({ extends: [[[[[{ rules: { semi: 2 } }]]]]] }) works and results in [{ rules: { semi: 2 } }].

Namespace normalization works only at the first level, though.

[mdjermanovic]

I think it would be good to check if this contains extends key and throw an error saying that nested extends are not allowed. Otherwise, end user would get a template message saying that flat config system doesn't support extends, which might be confusing since their top-level extends passed to defineConfig() are valid.

[nzakas]

Good point.