Ensure webpack build worker defaults on

[ijjk]

As discussed almost all webpack plugins shouldn't have issues with this being enabled by default so removing the default condition where we disable this optimization due to custom webpack config is more beneficial and in the rare case a custom plugin has issues it can still be manually disabled.

Closes NEXT-2523

[ijjk]

Stats from current PR

Default Build (Increase detected ⚠️)

General

	vercel/next.js canary	ijjk/next.js update/webpack-build-worker	Change
buildDuration	19.9s	19.9s	N/A
buildDurationCached	9s	7.8s	N/A
nodeModulesSize	196 MB	196 MB	N/A
nextStartRea..uration (ms)	426ms	426ms	✓

Client Bundles (main, webpack)

	vercel/next.js canary	ijjk/next.js update/webpack-build-worker	Change
1068-HASH.js gzip	30.3 kB	30.3 kB	N/A
3f784ff6-HASH.js gzip	53.5 kB	53.5 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	242 B	N/A
main-HASH.js gzip	32.1 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	ijjk/next.js update/webpack-build-worker	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	ijjk/next.js update/webpack-build-worker	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	ijjk/next.js update/webpack-build-worker	Change
_buildManifest.js gzip	485 B	484 B	N/A
Overall change	0 B	0 B	✓

Rendered Page Sizes

	vercel/next.js canary	ijjk/next.js update/webpack-build-worker	Change
index.html gzip	529 B	528 B	N/A
link.html gzip	541 B	541 B	✓
withRouter.html gzip	524 B	522 B	N/A
Overall change	541 B	541 B	✓

Edge SSR bundle Size

	vercel/next.js canary	ijjk/next.js update/webpack-build-worker	Change
edge-ssr.js gzip	94.5 kB	94.5 kB	N/A
page.js gzip	151 kB	151 kB	N/A
Overall change	0 B	0 B	✓

Middleware size

	vercel/next.js canary	ijjk/next.js update/webpack-build-worker	Change
middleware-b..fest.js gzip	624 B	627 B	N/A
middleware-r..fest.js gzip	151 B	151 B	✓
middleware.js gzip	44.6 kB	44.6 kB	N/A
edge-runtime..pack.js gzip	1.94 kB	1.94 kB	✓
Overall change	2.1 kB	2.1 kB	✓

Next Runtimes

	vercel/next.js canary	ijjk/next.js update/webpack-build-worker	Change
app-page-exp...dev.js gzip	166 kB	166 kB	✓
app-page-exp..prod.js gzip	95.9 kB	95.9 kB	✓
app-page-tur..prod.js gzip	97.6 kB	97.6 kB	✓
app-page-tur..prod.js gzip	92 kB	92 kB	✓
app-page.run...dev.js gzip	136 kB	136 kB	✓
app-page.run..prod.js gzip	90.6 kB	90.6 kB	✓
app-route-ex...dev.js gzip	22 kB	22 kB	✓
app-route-ex..prod.js gzip	14.9 kB	14.9 kB	✓
app-route-tu..prod.js gzip	14.9 kB	14.9 kB	✓
app-route-tu..prod.js gzip	14.6 kB	14.6 kB	✓
app-route.ru...dev.js gzip	21.7 kB	21.7 kB	✓
app-route.ru..prod.js gzip	14.6 kB	14.6 kB	✓
pages-api-tu..prod.js gzip	9.47 kB	9.47 kB	✓
pages-api.ru...dev.js gzip	9.74 kB	9.74 kB	✓
pages-api.ru..prod.js gzip	9.47 kB	9.47 kB	✓
pages-turbo...prod.js gzip	22.1 kB	22.1 kB	✓
pages.runtim...dev.js gzip	22.7 kB	22.7 kB	✓
pages.runtim..prod.js gzip	22.1 kB	22.1 kB	✓
server.runti..prod.js gzip	50.1 kB	50.1 kB	✓
Overall change	927 kB	927 kB	✓

build cache Overall increase ⚠️

	vercel/next.js canary	ijjk/next.js update/webpack-build-worker	Change
0.pack gzip	1.55 MB	1.55 MB	N/A
index.pack gzip	103 kB	104 kB	⚠️ +154 B
Overall change	103 kB	104 kB	⚠️ +154 B

Commit: 75e0664