chore: introduce prettier

[lumirlumir]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request?

In this PR, I've added Prettier to keep formatting consistent.

The prettier.config.js file follows the common setup used across the ESLint organization:

• https://github.com/eslint/markdown/blob/main/prettier.config.js
• https://github.com/eslint/css/blob/main/prettier.config.js
• https://github.com/eslint/json/blob/main/prettier.config.js
• https://github.com/eslint/rewrite/blob/main/prettier.config.js

I plan to add a .git-blame-ignore-revs file if preserving a clean git blame history is important.

What changes did you make? (Give an overview)

In this PR, I've added Prettier to keep formatting consistent.

Related Issues

Ref: #671

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

The git diff is quite large, but I haven't changed any logic. Occasionally eslint-disable or @ts-ignore comment directives were moved to the wrong position, I only adjusted them where necessary.

[lumirlumir]

This ignore pattern follows the same pattern used in eslint.config.js:

js/eslint.config.js

Lines 10 to 15 in 80033b8

	globalIgnores([
	"**/tests/fixtures/",
	"**/dist/",
	"**/coverage/",
	"packages/espree/tools/create-test-example.js"
	]),

[Copilot]

Pull request overview

This PR introduces Prettier for consistent code formatting across the ESLint JS monorepo. The configuration follows the standard setup used in other ESLint organization repositories.

Changes:

• Added Prettier configuration and tooling
• Formatted all source files with tabs, 4-space tab width, and arrow function parentheses avoidance
• Updated CI workflow to enforce formatting checks
• Removed eslint-config-eslint/formatting since Prettier now handles formatting

Reviewed changes

Copilot reviewed 89 out of 110 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
prettier.config.js	Adds Prettier configuration matching ESLint org standards
package.json	Adds Prettier dependency and formatting scripts
.prettierignore	Configures files to exclude from Prettier
eslint.config.js	Removes formatting ESLint config (delegated to Prettier)
.github/workflows/ci.yml	Adds formatting check to CI pipeline
packages/**/*.js	Applies consistent formatting with tabs and proper indentation
packages/**/*.{md,json,html,css}	Applies Prettier formatting to documentation and config files

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