Add a cjs build in exp

Author: hsubox76

packages/storage/exp/package.json

@@ -1,7 +1,7 @@
 {
   "name": "@firebase/storage/exp",
   "description": "A tree-shakeable version of the Storage SDK",
-  "main": "../dist/exp/index.browser.esm2017.js",
+  "main": "../dist/exp/index.browser.cjs.js",
   "module": "../dist/exp/index.browser.esm2017.js",
   "browser": "../dist/exp/index.browser.esm2017.js",
   "typings": "./dist/exp/index.d.ts",

packages/storage/rollup.config.exp.js

@@ -44,11 +44,18 @@ const plugins = [
 const browserBuilds = [
   {
     input: './exp/index.ts',
-    output: {
-      file: path.resolve('./exp', pkgExp.browser),
-      format: 'es',
-      sourcemap: true
-    },
+    output: [
+      {
+        file: path.resolve('./exp', pkgExp.main),
+        format: 'cjs',
+        sourcemap: true
+      },
+      {
+        file: path.resolve('./exp', pkgExp.browser),
+        format: 'es',
+        sourcemap: true
+      }
+    ],
     plugins,
     external: id => deps.some(dep => id === dep || id.startsWith(`${dep}/`)),
     treeshake: {