Dual packaging, without breaking existing consumers (#628)

Co-authored-by: Yasumasa Ashida <ys.ashida@gmail.com>

build/entries/bundled.js

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+/**
+ * UMD entry point for `bundled.js` — same as `es5.js`, but with the polyfills
+ * needed by the browser targets baked in.
+ *
+ * `core-js/stable` + `regenerator-runtime/runtime` is the core-js@3 equivalent
+ * of the deprecated `@babel/polyfill` that this bundle used to be built from.
+ * `@babel/preset-env`'s `useBuiltIns: 'entry'` rewrites the `core-js/stable`
+ * import below into just the polyfills the configured targets actually need.
+ *
+ * See `./es5.js` for why this only re-exports the default.
+ */
+import 'core-js/stable';
+import 'regenerator-runtime/runtime';
+import Bowser from '../../src/bowser.js';
+
+export default Bowser;

build/entries/es5.js

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+/**
+ * UMD entry point for `es5.js`.
+ *
+ * `src/bowser.js` also has named exports (`parse`, `getParser`) so that ESM
+ * consumers can `import { getParser } from 'bowser'`. A UMD bundle with named
+ * exports would expose `module.exports = { default, parse, getParser }`, which
+ * would break every existing `require('bowser')` and `window.bowser` consumer.
+ *
+ * Re-exporting only the default here keeps the UMD output at
+ * `module.exports = Bowser` / `window.bowser = Bowser`. The class carries
+ * `parse` and `getParser` as static methods, so nothing is lost.
+ */
+import Bowser from '../../src/bowser.js';
+
+export default Bowser;