chore: fix module metadata target

[YfengJ]

Summary

• update the package module field from ./dist/index.mjs to ./dist/index.js
• keep it aligned with the existing ESM export target in exports.import

Why

The published rollup-plugin-gzip@4.2.0 package declares module: "./dist/index.mjs", but the npm tarball does not include dist/index.mjs. It does include dist/index.js, which is already the package's ESM target through exports.import and exports.default.

This keeps legacy bundlers/tools that still read the module field from resolving a missing file.

Validation

• corepack yarn install --immutable
• corepack yarn run test
• corepack yarn run build
• corepack yarn run lint after build
• node -e "const fs=require('fs'); const pkg=require('./package.json'); const target=pkg.module.replace(/^\\.\\//,''); if(!fs.existsSync(target)){throw new Error('missing module target '+target)}; console.log(pkg.module+' exists')"
• npm pack --dry-run --ignore-scripts --json .
• git diff --check