add plugin to avoid too many css requests

[sokra]

What?

Reduces the number of css chunks per entrypoint by merging chunks.

Why?

In larger application the total CSS deduplication approach falls short in a sense of it's causing way too many separate chunks, which results in way to many requests. That affects page load performance.

Closes PACK-2598

[ijjk]

Stats from current PR

Default Build (Increase detected ⚠️)

General Overall increase ⚠️

	vercel/next.js canary	vercel/next.js sokra/css-merge-plugin	Change
buildDuration	16.7s	16.9s	⚠️ +194ms
buildDurationCached	9.2s	7.9s	N/A
nodeModulesSize	197 MB	197 MB	⚠️ +22.5 kB
nextStartRea..uration (ms)	482ms	493ms	N/A

Client Bundles (main, webpack)

	vercel/next.js canary	vercel/next.js sokra/css-merge-plugin	Change
1068-HASH.js gzip	30.3 kB	30.3 kB	N/A
3f784ff6-HASH.js gzip	53.7 kB	53.7 kB	N/A
4944-HASH.js gzip	5.04 kB	5.03 kB	N/A
8423.HASH.js gzip	181 B	181 B	✓
framework-HASH.js gzip	45.2 kB	45.2 kB	✓
main-app-HASH.js gzip	241 B	240 B	N/A
main-HASH.js gzip	32.2 kB	32.1 kB	N/A
webpack-HASH.js gzip	1.7 kB	1.7 kB	✓
Overall change	47.1 kB	47.1 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	vercel/next.js sokra/css-merge-plugin	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	vercel/next.js sokra/css-merge-plugin	Change
_app-HASH.js gzip	196 B	196 B	✓
_error-HASH.js gzip	184 B	183 B	N/A
amp-HASH.js gzip	503 B	504 B	N/A
css-HASH.js gzip	323 B	324 B	N/A
dynamic-HASH.js gzip	2.5 kB	2.51 kB	N/A
edge-ssr-HASH.js gzip	258 B	259 B	N/A
head-HASH.js gzip	353 B	351 B	N/A
hooks-HASH.js gzip	370 B	370 B	✓
image-HASH.js gzip	4.21 kB	4.2 kB	N/A
index-HASH.js gzip	259 B	259 B	✓
link-HASH.js gzip	2.68 kB	2.67 kB	N/A
routerDirect..HASH.js gzip	313 B	314 B	N/A
script-HASH.js gzip	386 B	385 B	N/A
withRouter-HASH.js gzip	309 B	311 B	N/A
1afbb74e6ecf..834.css gzip	106 B	106 B	✓
Overall change	931 B	931 B	✓

Client Build Manifests

	vercel/next.js canary	vercel/next.js sokra/css-merge-plugin	Change
_buildManifest.js gzip	485 B	484 B	N/A
Overall change	0 B	0 B	✓

Rendered Page Sizes

	vercel/next.js canary	vercel/next.js sokra/css-merge-plugin	Change
index.html gzip	528 B	528 B	✓
link.html gzip	540 B	540 B	✓
withRouter.html gzip	523 B	522 B	N/A
Overall change	1.07 kB	1.07 kB	✓

Edge SSR bundle Size

	vercel/next.js canary	vercel/next.js sokra/css-merge-plugin	Change
edge-ssr.js gzip	94.9 kB	94.9 kB	N/A
page.js gzip	3.06 kB	3.06 kB	N/A
Overall change	0 B	0 B	✓

Middleware size

	vercel/next.js canary	vercel/next.js sokra/css-merge-plugin	Change
middleware-b..fest.js gzip	626 B	627 B	N/A
middleware-r..fest.js gzip	151 B	151 B	✓
middleware.js gzip	25.4 kB	25.4 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 sokra/css-merge-plugin	Change
app-page-exp...dev.js gzip	170 kB	170 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	92.8 kB	92.8 kB	✓
app-page.run...dev.js gzip	151 kB	151 kB	✓
app-page.run..prod.js gzip	91.3 kB	91.3 kB	✓
app-route-ex...dev.js gzip	22 kB	22 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	14.7 kB	14.7 kB	✓
app-route.ru...dev.js gzip	21.7 kB	21.7 kB	✓
app-route.ru..prod.js gzip	14.7 kB	14.7 kB	✓
pages-api-tu..prod.js gzip	9.5 kB	9.5 kB	✓
pages-api.ru...dev.js gzip	9.78 kB	9.78 kB	✓
pages-api.ru..prod.js gzip	9.5 kB	9.5 kB	✓
pages-turbo...prod.js gzip	22.3 kB	22.3 kB	✓
pages.runtim...dev.js gzip	23 kB	23 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	950 kB	950 kB	✓

build cache Overall increase ⚠️

	vercel/next.js canary	vercel/next.js sokra/css-merge-plugin	Change
0.pack gzip	1.56 MB	1.55 MB	N/A
index.pack gzip	104 kB	105 kB	⚠️ +836 B
Overall change	104 kB	105 kB	⚠️ +836 B

Commit: 8888504

[ijjk]

Tests Passed