fix: wretch default export compatibility in CommonJS (#32)

Author: thanhdanh

wretch.js

@@ -1,5 +1,5 @@
 // NOTE: does not export Wretcher
-const wretch = require('./wretch.browser')
+const { default: wretch } = require('./wretch.browser')
 const fetch = require('./fetch')
 
 wretch().polyfills({ fetch })