fix: add ESM exports to support consistent CJS interop in Vite/Rolldown

[neisukee]

Problem

With the introduction of consistent CJS interop in Vite (powered by Rolldown),
projects using "type": "module" receive module.exports as the default import
instead of module.exports.default.

This causes react-ga4's default import to return the entire exports object
rather than the GA4 instance, breaking ga4.initialize() and other methods.

See: https://rolldown.rs/in-depth/bundling-cjs#ambiguous-default-import-from-cjs-modules

Solution

Add an ESM entry point via the exports field in package.json,
so bundlers can resolve the proper ESM version directly.

Changes

• Modified babel.config.js to support ESM build (modules: false when BABEL_ENV=esm)
• Added .js extensions to relative imports in src/ for Node.js ESM compatibility
• CJS build output moved to dist/cjs/
• ESM build output added at dist/esm/
• Added exports, module fields to package.json