Set 'react' and 'react-dom' as externals for browserify

When browserifying anything that depends on React, don't bundle another
copy of React in the file. There might be some other common dependencies
that we should include here.

scripts/browserify.js

@@ -39,7 +39,11 @@ async function getBrowserify(signatures) {
 					bundleFs
 					.on('error', reject)
 					.on('finish', resolve);
-					browserify(f, config.config).bundle().pipe(bundleFs);
+					browserify(f, config.config)
+						.external('react')
+						.external('react-dom')
+						.bundle()
+						.pipe(bundleFs);
 				});
 
 				onProgress(f, dest, 'browserify');