Add missing isReady declaration to NextRouter type

[webda2l]

Follow #20628

[ijjk]

Stats from current PR

Default Server Mode

General Overall increase ⚠️

	vercel/next.js canary	webda2l/next.js patch-1	Change
buildDuration	8.7s	8.8s	⚠️ +120ms
nodeModulesSize	80.8 MB	80.8 MB	⚠️ +29 B

Page Load Tests Overall increase ✓

	vercel/next.js canary	webda2l/next.js patch-1	Change
/ failed reqs	0	0	✓
/ total time (seconds)	1.728	1.741	⚠️ +0.01
/ avg req/sec	1446.61	1436.23	⚠️ -10.38
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.11	1.103	-0.01
/error-in-render avg req/sec	2252.25	2266.08	+13.83

Client Bundles (main, webpack, commons)

	vercel/next.js canary	webda2l/next.js patch-1	Change
677f882d2ed8..89aa.js gzip	13.1 kB	13.1 kB	✓
framework.HASH.js gzip	39 kB	39 kB	✓
main-8b99f8a..8ccf.js gzip	6.63 kB	6.63 kB	✓
webpack-50be..df5b.js gzip	751 B	751 B	✓
Overall change	59.4 kB	59.4 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	webda2l/next.js patch-1	Change
polyfills-81..14d7.js gzip	31.2 kB	31.2 kB	✓
Overall change	31.2 kB	31.2 kB	✓

Client Pages

	vercel/next.js canary	webda2l/next.js patch-1	Change
_app-b6fc6bc..222c.js gzip	1.28 kB	1.28 kB	✓
_error-e2ffa..0f3f.js gzip	3.46 kB	3.46 kB	✓
hooks-010c20..8411.js gzip	887 B	887 B	✓
index-bbee2f..528b.js gzip	227 B	227 B	✓
link-d979103..c5ff.js gzip	1.64 kB	1.64 kB	✓
routerDirect..bf84.js gzip	303 B	303 B	✓
withRouter-a..5826.js gzip	302 B	302 B	✓
Overall change	8.09 kB	8.09 kB	✓

Client Build Manifests

	vercel/next.js canary	webda2l/next.js patch-1	Change
_buildManifest.js gzip	323 B	323 B	✓
Overall change	323 B	323 B	✓

Rendered Page Sizes

	vercel/next.js canary	webda2l/next.js patch-1	Change
index.html gzip	613 B	613 B	✓
link.html gzip	618 B	618 B	✓
withRouter.html gzip	606 B	606 B	✓
Overall change	1.84 kB	1.84 kB	✓

---

Serverless Mode

General Overall increase ⚠️

	vercel/next.js canary	webda2l/next.js patch-1	Change
buildDuration	10.7s	10.3s	-364ms
nodeModulesSize	80.8 MB	80.8 MB	⚠️ +29 B

Client Bundles (main, webpack, commons)

	vercel/next.js canary	webda2l/next.js patch-1	Change
677f882d2ed8..89aa.js gzip	13.1 kB	13.1 kB	✓
framework.HASH.js gzip	39 kB	39 kB	✓
main-8b99f8a..8ccf.js gzip	6.63 kB	6.63 kB	✓
webpack-50be..df5b.js gzip	751 B	751 B	✓
Overall change	59.4 kB	59.4 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	webda2l/next.js patch-1	Change
polyfills-81..14d7.js gzip	31.2 kB	31.2 kB	✓
Overall change	31.2 kB	31.2 kB	✓

Client Pages

	vercel/next.js canary	webda2l/next.js patch-1	Change
_app-b6fc6bc..222c.js gzip	1.28 kB	1.28 kB	✓
_error-e2ffa..0f3f.js gzip	3.46 kB	3.46 kB	✓
hooks-010c20..8411.js gzip	887 B	887 B	✓
index-bbee2f..528b.js gzip	227 B	227 B	✓
link-d979103..c5ff.js gzip	1.64 kB	1.64 kB	✓
routerDirect..bf84.js gzip	303 B	303 B	✓
withRouter-a..5826.js gzip	302 B	302 B	✓
Overall change	8.09 kB	8.09 kB	✓

Client Build Manifests

	vercel/next.js canary	webda2l/next.js patch-1	Change
_buildManifest.js gzip	323 B	323 B	✓
Overall change	323 B	323 B	✓

Serverless bundles

	vercel/next.js canary	webda2l/next.js patch-1	Change
_error.js	1 MB	1 MB	✓
404.html	2.67 kB	2.67 kB	✓
hooks.html	1.92 kB	1.92 kB	✓
index.js	1 MB	1 MB	✓
link.js	1.06 MB	1.06 MB	✓
routerDirect.js	1.05 MB	1.05 MB	✓
withRouter.js	1.05 MB	1.05 MB	✓
Overall change	5.18 MB	5.18 MB	✓

Commit: f8e3e5b