Rollup JS files with rollup-typescript-plugin (#8503)

The rollup-typescript-plugin does not transpile JS files using the TS
compiler by default. This means that external dependencies that provide
JS bundles will not be transpiled to the target ES version specified in
the TypeScript config used by the plugin.

This resulted in one of our dependencies (https://github.com/jakearchibald/idb)
being included in the CDN bundles without being transpiled to
ES5 (the target ES version). Since this dependencies bundle uses
ES2018 syntax (object spread operator `{ ...x }`), this upgraded our CDN
bundles' minimum ES version requirement to ES2018 which isn't compatible with older browser versions.
To see the ES2018 syntax in one of the CDN bundles, see
https://www.gstatic.com/firebasejs/10.13.1/firebase-app.js and search for `...oldTraps`.

Author: dlarocque

packages/firebase/rollup.config.js

@@ -36,6 +36,8 @@ const typescriptPlugin = rollupTypescriptPlugin({
 
 const typescriptPluginCDN = rollupTypescriptPlugin({
   typescript,
+  allowJs: true,
+  include: ['*.ts', '**/*.ts', '*.js', '**/*.js'],
   tsconfigOverride: {
     compilerOptions: {
       declaration: false