feat: add es5 build output

fixes #60

Author: Soc Sieng

rollup.config.js

@@ -53,6 +53,31 @@ export default [
     external: ['react'],
     plugins: [rollupJson(), typescript()],
   },
+  {
+    input: 'src/button-react/index.ts',
+    output: [
+      {
+        file: 'src/button-react/dist/index.es5.min.js',
+        format: 'umd',
+        name: 'GooglePayButton',
+        globals: {
+          react: 'React',
+        },
+        plugins: [terser()],
+      },
+    ],
+    external: ['react'],
+    plugins: [
+      rollupJson(),
+      typescript({
+        tsconfigOverride: {
+          compilerOptions: {
+            target: 'es5',
+          },
+        },
+      }),
+    ],
+  },
   {
     input: 'src/button-react/index.ts',
     output: {
@@ -90,6 +115,27 @@ export default [
     ],
     plugins: [rollupJson(), typescript()],
   },
+  {
+    input: 'src/button-element/index.ts',
+    output: [
+      {
+        file: 'src/button-element/dist/index.es5.min.js',
+        format: 'umd',
+        name: 'GooglePayButton',
+        plugins: [terser()],
+      },
+    ],
+    plugins: [
+      rollupJson(),
+      typescript({
+        tsconfigOverride: {
+          compilerOptions: {
+            target: 'es5',
+          },
+        },
+      }),
+    ],
+  },
   {
     input: 'src/button-element/index.ts',
     output: {