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: eslint/js#671

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

N/A

[Copilot]

Pull request overview

This PR introduces Prettier as the code formatter for the eslint-transforms repository to ensure consistent formatting across the codebase. The configuration follows the standard setup used across other ESLint organization repositories.

Changes:

• Added Prettier 3.8.1 as a dev dependency with configuration matching other ESLint org repos
• Reformatted all JavaScript files to use tabs (tabWidth: 4) and JSON files to use 2 spaces
• Removed ESLint formatting configuration in favor of Prettier
• Added new CI workflow job to check formatting
• Created .prettierignore to exclude CHANGELOG.md and test fixtures

Reviewed changes

Copilot reviewed 11 out of 15 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
prettier.config.js	New Prettier configuration file with tabs for JS, spaces for JSON
.prettierignore	Excludes CHANGELOG.md and test fixtures from formatting
package.json	Adds Prettier 3.8.1 dependency and fmt/fmt:check scripts; updates ESLint version
eslint.config.js	Removes eslint-config-eslint/formatting, reformatted with tabs
.github/workflows/ci.yml	Adds new "format" job to check Prettier formatting, reformatted
.github/workflows/release-please.yml	Reformatted with consistent YAML indentation
tests/**/*.js	Reformatted with tabs instead of spaces
lib/**/*.js	Reformatted with tabs instead of spaces
bin/eslint-transforms.js	Reformatted with tabs instead of spaces
README.md	Reformatted code examples with tabs
release-please-config.json	Reformatted with 2-space indentation
.release-please-manifest.json	Reformatted with 2-space indentation

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