update to postcss v8

[MoOx]

This required a big change to work with v8.

Closes #25

[Copilot]

Pull request overview

This PR modernizes postcss-filter-plugins to work with PostCSS v8 and aligns dependency management with the request in #25 by making PostCSS a peer dependency, while also updating the project’s build/test/tooling to a simpler Node-based setup.

Changes:

• Migrates the plugin implementation to the PostCSS 8 plugin API (postcssPlugin + prepare).
• Moves PostCSS to peerDependencies (and keeps it in devDependencies for local development/testing).
• Replaces AVA-based tests with Node’s built-in test runner and refreshes repository metadata/docs.

Reviewed changes

Copilot reviewed 10 out of 13 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/index.js	Rewrites the plugin implementation for PostCSS 8 (new plugin API + duplicate filtering logic).
src/index.test.js	Adds a new Node node:test suite covering duplicate filtering behavior.
src/tests/index.js	Removes the previous AVA-based test suite.
README.md	Updates README formatting and examples (but still contains legacy PostCSS plugin API usage).
package.json	Switches to peerDependency for PostCSS, updates scripts/tooling, and changes published entry point to src/.
LICENSE	Adds MIT license text as LICENSE.
CHANGELOG.md	Normalizes markdown list formatting.
.node-version	Pins a Node major version for development.
.gitignore	Removes dist ignore since dist/ is no longer produced/used.
.editorconfig	Removes EditorConfig configuration.
.babelrc	Removes Babel configuration (no longer building dist/).
.all-contributorsrc	Removes all-contributors configuration.

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