fix(rollup-plugin): point module field to emitted ESM file

[itosa-kazu]

Summary

rollup-plugin-purgecss currently advertises the ESM entry as ./lib/rollup-plugin-purgecss.es.js, but the built package emits and publishes lib/rollup-plugin-purgecss.esm.js.

This updates both module and jsnext:main to point at the emitted ESM file.

Validation

• npm ci --ignore-scripts
• npm run build --workspace rollup-plugin-purgecss
• npm pack --workspace rollup-plugin-purgecss --pack-destination $env:TEMP --json --ignore-scripts
• Installed the packed tarball in a clean temp project and confirmed both module and main point at files present in the package.

npm test --workspace rollup-plugin-purgecss -- --runInBand currently fails while parsing the package's existing jest.config.ts, before tests run, because it cannot resolve the root jest.config import in this environment.