test: ensure @eslint/markdown works with both ESLint v9 and v10

[lumirlumir]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request?

This PR was motivated by eslint/json#187 (comment).

Currently, when updating @eslint/core to v1.0.0, CI fails because @eslint/core@1.0.0 does not match ESLint v9

@eslint/core@1.0.0 includes type information for ESLint v10, which causes the type mismatch.

So, I've updated ESLint to >=10.0.0-alpha.0 <10.0.0 || ^10.0.1. (Ref: https://github.com/eslint/eslint/blob/main/packages/eslint-config-eslint/package.json#L66)

---

When I update ESLint to >=10.0.0-alpha.0 <10.0.0 || ^10.0.1, the following error occurs, so I added the legacy-peer-deps option to .npmrc.

This change follows the approach in eslint/eslint#20281

What changes did you make? (Give an overview)

In this PR, I've updated @eslint/core, add legacy-peer-deps, and use ESLint v10 pre-release.

Related Issues

Fixes: #621
Ref: #584, #586, eslint/json#187 (comment), eslint/eslint#20281

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

N/A

[lumirlumir]

Unlike the PRs in JSON and CSS, the Markdown plugin needs further investigation to support ESLint v10. I'm marking this as a draft for now and will investigate it soon.

[mdjermanovic]

Same question as eslint/json#190 (comment). We can clarify there, just wanted to leave a note here as well so that we don't merge until this is clarified.

[lumirlumir]

Currently, both eslint-plugin-react and typescript-eslint do not yet support ESLint v10 as a peer dependency, I've used the latest v9 versions.

[lumirlumir]

The test strategy follows the CSS and JSON.

• CSS: https://github.com/eslint/css/blob/main/.github/workflows/ci.yml
• JSON: https://github.com/eslint/json/blob/main/.github/workflows/ci.yml

[Copilot]

Pull request overview

Updates the package and test harness to work with the newer @eslint/* packages and to validate compatibility across ESLint v9 and v10.

Changes:

• Bump @eslint/core and @eslint/plugin-kit and adjust type tests to use @eslint/core’s Plugin type.
• Update test code to use ESLint directly, lazily import LegacyESLint, and skip legacy-only tests on ESLint v10+.
• Expand CI to run tests/typechecks against multiple ESLint versions (including 10.x).

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
package.json	Updates @eslint/core / @eslint/plugin-kit versions to align with ESLint v10 types/tooling.
tests/types/types.test.ts	Updates type assertions to use Plugin from @eslint/core instead of ESLint.Plugin.
tests/plugin.test.js	Refactors ESLint imports/initialization for v10, adds version gating for legacy tests, and makes LegacyESLint import lazy.
tests/examples/all.test.js	Switches example validation from FlatESLint to ESLint.
src/index.js	Uses JSDoc @satisfies for processorRulesConfig typing.
examples/react/package.json	Updates example devDependencies (ESLint/@eslint/js versions).
examples/typescript/package.json	Updates example devDependencies (ESLint/@eslint/js/typescript/typescript-eslint versions).
.github/workflows/ci.yml	Expands the ESLint version matrix and installs ESLint per-matrix entry for tests and type checks.

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[lumirlumir]

While working on this, I've found a v9 type compatibility issue, so I'm marking this as a draft until a decision is reached in eslint/json#213.

I used the JSON plugin as an example because newer major versions of CSS and Markdown haven't been released yet.

Note to myself: This PR also need to address #605 (comment).

[bricss]

Perhaps this nickdeis/eslint-plugin-no-secrets#44 may help 🚑 with fixes 🪛

[bricss]

Note: Currently, the only way to make Markdown plugin 🔌 work with ESLint v10 is to use the following config:

import markdown from '@eslint/markdown';
import { defineConfig } from 'eslint/config';

export default defineConfig([
  ...markdown.configs.processor,
]);

[lumirlumir]

I've followed the same pattern used for the ESLint v9 jiti library installation.

Ref: https://github.com/eslint/eslint/blob/v9.39.3/package.json#L178-L179

This also addresses the comment mentioned in #605 (comment).