Add basePath flag

[ph1p]

When a Next.js app uses a basePath (e.g., /dashboard) the proxy.js did not load and shows a 404.
Now its fixed.

[adriaandotcom]

Thanks a lot!

Is basePath always set up? Aka, do we need a fallback for any version?

[ph1p]

All previous versions should work without changes. The default value of rewrites.basePath is true. The problem occurs with the tag <Script src="/proxy.js"/>. If we define a basePath (e.g., /dashboard), the proxy.js file cannot be found because the path would now have to be /dashboard/proxy.js. To prevent this, we simply set rewrites.basePath to false when a basePath is specified.