Provide default fallback _document and _app for for concurrent mode

[huozhi]

• if _app is not provided, fallback to default _app page
• If _document is not provided, fallback to inline functional components version or use the default
• if Document gIP is provided, error

Closes #30654

[shuding]

Thanks!

[ijjk]

Stats from current PR

Default Build (Decrease detected ✓)

General Overall increase ⚠️

	vercel/next.js canary	huozhi/next.js rsc/fallback-page	Change
buildDuration	18.3s	18.2s	-126ms
buildDurationCached	3.9s	3.9s	⚠️ +41ms
nodeModulesSize	294 MB	294 MB	⚠️ +2.07 kB

Page Load Tests Overall decrease ⚠️

	vercel/next.js canary	huozhi/next.js rsc/fallback-page	Change
/ failed reqs	0	0	✓
/ total time (seconds)	2.837	2.901	⚠️ +0.06
/ avg req/sec	881.27	861.63	⚠️ -19.64
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.337	1.338	0
/error-in-render avg req/sec	1869.47	1867.77	⚠️ -1.7

Client Bundles (main, webpack, commons)

	vercel/next.js canary	huozhi/next.js rsc/fallback-page	Change
450.HASH.js gzip	179 B	179 B	✓
framework-HASH.js gzip	42.2 kB	42.2 kB	✓
main-HASH.js gzip	28 kB	28 kB	✓
webpack-HASH.js gzip	1.45 kB	1.45 kB	✓
Overall change	71.9 kB	71.9 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	huozhi/next.js rsc/fallback-page	Change
polyfills-a4..dd70.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	huozhi/next.js rsc/fallback-page	Change
_app-HASH.js gzip	1.23 kB	1.23 kB	✓
_error-HASH.js gzip	194 B	194 B	✓
amp-HASH.js gzip	312 B	312 B	✓
css-HASH.js gzip	327 B	327 B	✓
dynamic-HASH.js gzip	2.38 kB	2.38 kB	✓
head-HASH.js gzip	350 B	350 B	✓
hooks-HASH.js gzip	635 B	635 B	✓
image-HASH.js gzip	4.44 kB	4.44 kB	✓
index-HASH.js gzip	263 B	263 B	✓
link-HASH.js gzip	1.87 kB	1.87 kB	✓
routerDirect..HASH.js gzip	321 B	321 B	✓
script-HASH.js gzip	383 B	383 B	✓
withRouter-HASH.js gzip	318 B	318 B	✓
334f979574ae..6f4.css gzip	106 B	106 B	✓
Overall change	13.1 kB	13.1 kB	✓

Client Build Manifests

	vercel/next.js canary	huozhi/next.js rsc/fallback-page	Change
_buildManifest.js gzip	459 B	459 B	✓
Overall change	459 B	459 B	✓

Rendered Page Sizes

	vercel/next.js canary	huozhi/next.js rsc/fallback-page	Change
index.html gzip	533 B	533 B	✓
link.html gzip	546 B	546 B	✓
withRouter.html gzip	527 B	527 B	✓
Overall change	1.61 kB	1.61 kB	✓

---

Default Build with SWC (Decrease detected ✓)

General Overall increase ⚠️

	vercel/next.js canary	huozhi/next.js rsc/fallback-page	Change
buildDuration	19.7s	19.7s	⚠️ +9ms
buildDurationCached	3.9s	3.9s	-41ms
nodeModulesSize	294 MB	294 MB	⚠️ +2.07 kB

Page Load Tests Overall decrease ⚠️

	vercel/next.js canary	huozhi/next.js rsc/fallback-page	Change
/ failed reqs	0	0	✓
/ total time (seconds)	2.916	2.818	-0.1
/ avg req/sec	857.23	887.04	+29.81
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.302	1.352	⚠️ +0.05
/error-in-render avg req/sec	1920.07	1849.55	⚠️ -70.52

Client Bundles (main, webpack, commons)

	vercel/next.js canary	huozhi/next.js rsc/fallback-page	Change
450.HASH.js gzip	179 B	179 B	✓
framework-HASH.js gzip	42.3 kB	42.3 kB	✓
main-HASH.js gzip	28.2 kB	28.2 kB	✓
webpack-HASH.js gzip	1.43 kB	1.43 kB	✓
Overall change	72.1 kB	72.1 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	huozhi/next.js rsc/fallback-page	Change
polyfills-a4..dd70.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	huozhi/next.js rsc/fallback-page	Change
_app-HASH.js gzip	1.22 kB	1.22 kB	✓
_error-HASH.js gzip	180 B	180 B	✓
amp-HASH.js gzip	305 B	305 B	✓
css-HASH.js gzip	321 B	321 B	✓
dynamic-HASH.js gzip	2.38 kB	2.38 kB	✓
head-HASH.js gzip	342 B	342 B	✓
hooks-HASH.js gzip	622 B	622 B	✓
image-HASH.js gzip	4.46 kB	4.46 kB	✓
index-HASH.js gzip	256 B	256 B	✓
link-HASH.js gzip	1.91 kB	1.91 kB	✓
routerDirect..HASH.js gzip	314 B	314 B	✓
script-HASH.js gzip	375 B	375 B	✓
withRouter-HASH.js gzip	309 B	309 B	✓
334f979574ae..6f4.css gzip	106 B	106 B	✓
Overall change	13.1 kB	13.1 kB	✓

Client Build Manifests

	vercel/next.js canary	huozhi/next.js rsc/fallback-page	Change
_buildManifest.js gzip	460 B	460 B	✓
Overall change	460 B	460 B	✓

Rendered Page Sizes

	vercel/next.js canary	huozhi/next.js rsc/fallback-page	Change
index.html gzip	534 B	534 B	✓
link.html gzip	547 B	547 B	✓
withRouter.html gzip	528 B	528 B	✓
Overall change	1.61 kB	1.61 kB	✓

Commit: 672daeb