refactor(error-overlay): unify Pages/App router error overlay source

[balazsorban44]

This is a further iteration on #60899 which first moved the Pages error overlay from a different package into the next directly. While working on #62942 I noticed that a lot of the logic is shared between the two routers' error overlay implementation.

A lot of type/code duplication can be spared, using the https://github.com/vercel/next.js/blob/feat/unified-app-pages-error-overlay/packages/next/src/client/components/react-dev-overlay/shared.ts file.

Since there is no significant difference between the HMR events between Pages/App, it's easier to maintain a single shared implementation than remembering to sync changes in multiple places.

I also moved the Pages overlay-related changes into the same level where App Router's overlay is defined, so the file structure is easier to traverse/understand.

[ijjk]

Tests Passed

[ijjk]

Stats from current PR

Default Build (Increase detected ⚠️)

General

	vercel/next.js canary	vercel/next.js feat/unified-app-pages-error-overlay	Change
buildDuration	13.6s	13.7s	⚠️ +145ms
buildDurationCached	8.3s	6.6s	N/A
nodeModulesSize	198 MB	198 MB	N/A
nextStartRea..uration (ms)	428ms	437ms	N/A

Client Bundles (main, webpack)

	vercel/next.js canary	vercel/next.js feat/unified-app-pages-error-overlay	Change
2453-HASH.js gzip	30.5 kB	30.5 kB	N/A
3304.HASH.js gzip	181 B	181 B	✓
3f784ff6-HASH.js gzip	53.7 kB	53.7 kB	N/A
8299-HASH.js gzip	5.04 kB	5.04 kB	N/A
framework-HASH.js gzip	45.2 kB	45.2 kB	✓
main-app-HASH.js gzip	241 B	243 B	N/A
main-HASH.js gzip	32.2 kB	32.2 kB	N/A
webpack-HASH.js gzip	1.68 kB	1.68 kB	N/A
Overall change	45.4 kB	45.4 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	vercel/next.js feat/unified-app-pages-error-overlay	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	vercel/next.js feat/unified-app-pages-error-overlay	Change
_app-HASH.js gzip	196 B	197 B	N/A
_error-HASH.js gzip	184 B	184 B	✓
amp-HASH.js gzip	505 B	505 B	✓
css-HASH.js gzip	324 B	325 B	N/A
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edge-ssr-HASH.js gzip	258 B	258 B	✓
head-HASH.js gzip	352 B	352 B	✓
hooks-HASH.js gzip	370 B	371 B	N/A
image-HASH.js gzip	4.21 kB	4.21 kB	✓
index-HASH.js gzip	259 B	259 B	✓
link-HASH.js gzip	2.67 kB	2.67 kB	N/A
routerDirect..HASH.js gzip	314 B	312 B	N/A
script-HASH.js gzip	386 B	386 B	✓
withRouter-HASH.js gzip	309 B	309 B	✓
1afbb74e6ecf..834.css gzip	106 B	106 B	✓
Overall change	6.57 kB	6.57 kB	✓

Client Build Manifests

	vercel/next.js canary	vercel/next.js feat/unified-app-pages-error-overlay	Change
_buildManifest.js gzip	481 B	484 B	N/A
Overall change	0 B	0 B	✓

Rendered Page Sizes

	vercel/next.js canary	vercel/next.js feat/unified-app-pages-error-overlay	Change
index.html gzip	529 B	528 B	N/A
link.html gzip	541 B	541 B	✓
withRouter.html gzip	524 B	523 B	N/A
Overall change	541 B	541 B	✓

Edge SSR bundle Size

	vercel/next.js canary	vercel/next.js feat/unified-app-pages-error-overlay	Change
edge-ssr.js gzip	95.1 kB	95.1 kB	N/A
page.js gzip	3.06 kB	3.07 kB	N/A
Overall change	0 B	0 B	✓

Middleware size

	vercel/next.js canary	vercel/next.js feat/unified-app-pages-error-overlay	Change
middleware-b..fest.js gzip	626 B	623 B	N/A
middleware-r..fest.js gzip	151 B	151 B	✓
middleware.js gzip	25.5 kB	25.5 kB	N/A
edge-runtime..pack.js gzip	839 B	839 B	✓
Overall change	990 B	990 B	✓

Next Runtimes

	vercel/next.js canary	vercel/next.js feat/unified-app-pages-error-overlay	Change
app-page-exp...dev.js gzip	171 kB	171 kB	✓
app-page-exp..prod.js gzip	96.5 kB	96.5 kB	✓
app-page-tur..prod.js gzip	98.3 kB	98.3 kB	✓
app-page-tur..prod.js gzip	92.7 kB	92.7 kB	✓
app-page.run...dev.js gzip	149 kB	149 kB	✓
app-page.run..prod.js gzip	91.2 kB	91.2 kB	✓
app-route-ex...dev.js gzip	21.3 kB	21.3 kB	✓
app-route-ex..prod.js gzip	15 kB	15 kB	✓
app-route-tu..prod.js gzip	15 kB	15 kB	✓
app-route-tu..prod.js gzip	14.8 kB	14.8 kB	✓
app-route.ru...dev.js gzip	21 kB	21 kB	✓
app-route.ru..prod.js gzip	14.8 kB	14.8 kB	✓
pages-api-tu..prod.js gzip	9.52 kB	9.52 kB	✓
pages-api.ru...dev.js gzip	9.8 kB	9.8 kB	✓
pages-api.ru..prod.js gzip	9.52 kB	9.52 kB	✓
pages-turbo...prod.js gzip	22.3 kB	22.3 kB	✓
pages.runtim...dev.js gzip	22.9 kB	22.9 kB	✓
pages.runtim..prod.js gzip	22.3 kB	22.3 kB	✓
server.runti..prod.js gzip	50.5 kB	50.5 kB	✓
Overall change	948 kB	948 kB	✓

build cache Overall increase ⚠️

	vercel/next.js canary	vercel/next.js feat/unified-app-pages-error-overlay	Change
0.pack gzip	1.56 MB	1.56 MB	⚠️ +584 B
index.pack gzip	105 kB	105 kB	N/A
Overall change	1.56 MB	1.56 MB	⚠️ +584 B

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Commit: 78c2f13

[kdy1]

(I reviewed to learn about source code related to error overlay, as I'm working on PACK-2405)