chore: Install tsdown and create a dual package for UMD and ESM

tsdown.config.ts

@@ -0,0 +1,24 @@
+import { defineConfig } from 'tsdown';
+
+const banner = `Bowser - a browser detector
+https://github.com/lancedikson/bowser
+MIT License | (c) Dustin Diaz 2012-2015
+MIT License | (c) Denis Demchenko 2015-2026`;
+
+export default defineConfig([
+  {
+    entry: ['src/bowser.js'],
+    format: ['umd'],
+    outDir: 'dist/umd', // universal module definition
+    globalName: 'Bowser',
+    dts: false,
+    clean: true,
+  },
+  {
+    entry: ['src/bowser.js'],
+    format: ['esm'],
+    outDir: 'dist/esm', // es module
+    dts: false,
+    clean: true
+  },
+])