feat: update to use changesets

[castastrophe]

What

• Migrate the plugin to native ESM ("type": "module", import/export, node: core imports)
• Rename the package to the @allons-y/postcss-licensing scope
• Replace semantic-release/commitlint with Changesets, and rewrite the release workflow around it (npm OIDC trusted publishing, auto-tagging)
• Move ESLint to flat config with JS/JSON/JSONC/Markdown linting; refresh Renovate config, .gitignore, Husky hooks, and bump Yarn to 4.17.1

Why

• PostCSS 8 itself moved to ESM; this keeps the plugin aligned and drops CJS/ESM interop overhead for consumers
• Changesets adds a per-PR changelog entry and a final review step before versions/tags are cut
• The scope rename and tooling refresh bring this package in line with allonsy-studio's other project templates

Breaking changes

• No CommonJS entry point — consumers must import instead of require()
• Package name changes from postcss-licensing to @allons-y/postcss-licensing

[changeset-bot]

🦋 Changeset detected

Latest commit: 7ed6e15

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@allons-y/postcss-licensing	Major

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[github-actions]

Package	Line Rate	Branch Rate	Complexity	Health
main	100%	95%	0	✔
Summary	100% (79 / 79)	95% (21 / 22)	0	✔