feat!: support flat config

[aladdin-add]

Implementing flat support is a good opportunity that allows us to rewrite and simplify its implementation. This PR implements an initial version for gathering feedback. There are some challenges and issues:

1. Some plugins temporarily do not support flat config. Should we consider using @eslint/eslintrc for compatibility?
2. env: node/browser is no longer supported in flat config. Should we introduce the globals package to support this feature?
3. --config was previously used as a shortcut for using shareable configurations. There seems to be no reliable way to determine if it is in flat format. Can we assume that it is?

fixes #51

Remaining tasks:

• vue.js support
• ts support
• --config options
• docs updates

🧪for testing

$ npm init @weiran.zsd/eslint-config@latest

[nzakas]

1. Some plugins temporarily do not support flat config. Should we consider using `@eslint/eslintrc` for compatibility?

If they work correctly with FlatCompat, then yes, I think this makes sense.

2. `env: node/browser` is no longer supported in flat config. Should we introduce the `globals` package to support this feature?

Yes.

3. `--config` was previously used as a shortcut for using shareable configurations. There seems to be no reliable way to determine if it is in flat format. Can we assume that it is?

If the CLI is running in flat config mode, then --config will assume the file is in flat config format.

[nzakas]

@aladdin-add what's the status on this? I'd really like to roll out this change soon, so if you need help, please us know.

[aladdin-add]

@nzakas I've added the remaining tasks in the PR desc.

[nzakas]

Thanks, and do you need help finishing? As I mentioned, we'd like to get this out ASAP.

[aladdin-add]

The main blocker is many community plugins/configs do not support it yet. (vue, airbnb, xo as of now).

eslint/eslint#18093

I'd like it to be officially supported before we merge this.

[nzakas]

We can't really afford to wait for all plugins to be updated because v9.0.0 is already in beta and it doesn't make sense to have us generating eslintrc files anymore. Can't we just use FlatCompat for now?

[fasttime]

LGTM, thanks! Leaving open for @mdjermanovic to review, as requested.

[aladdin-add]

friendly ping @mdjermanovic

[aladdin-add]

eslint-plugin-vue has landed the official supports: https://github.com/vuejs/eslint-plugin-vue/releases/tag/v9.24.0

[mdjermanovic]

Is it intentional that we no longer show the list of dependencies that are going to be installed and ask the user for confirmation to install them?

[aladdin-add]

yes, it's relying on installing to update the package.json, and it seems not especially useful. I can add it back if you like it tho. :)

[mdjermanovic]

yes, it's relying on installing to update the package.json, and it seems not especially useful. I can add it back if you like it tho. :)

I think we should add it back to let the user know what packages are going to be installed and provide a way to generate the config file without installing them (e.g., in case the user is using some package manager that isn't listed).

[nzakas]

I agree, I think showing the dependencies to install before installing them is useful and should be included.

[aladdin-add]

@mdjermanovic I've pushed a commit: 0a4c547

[nzakas]

LGTM. Leaving open for @mdjermanovic to verify.

[mdjermanovic]

LGTM, thanks!