Add missing isReady declaration to NextRouter type (#20819)
Add missing isReady declaration to NextRouter type (#20819)

Follow https://github.com/vercel/next.js/pull/20628
JeromeFitz/next.js,zeit/next.js,zeit/next.js,azukaru/next.js,azukaru/next.js,zeit/next.js,JeromeFitz/next.js,flybayer/next.js,azukaru/next.js,flybayer/next.js,JeromeFitz/next.js,flybayer/next.js,azukaru/next.js,JeromeFitz/next.js,flybayer/next.js