perf: enable webpack build worker

[feedthejim]

This PR enables the experimental.webpackBuildWorker to be on by default. This flag enables logic inside Next.js to run the compilation in an isolated worker. The reason for this is that the webpack compilation process retains a lot of memory for the whole duration of the build process because it uses some packages that leak. We don't need it for the rest of the process so it's best to use it in a worker and leave the memory to be used for static generation.

This will improve memory usage during build, avoiding OOMs caused by webpack exceeding memory.

[ijjk]

Tests Passed

[ijjk]

Stats from current PR

Default Build

General Overall increase ⚠️

	vercel/next.js canary	vercel/next.js feedthejim/enable-build-worker	Change
buildDuration	10.1s	10.4s	⚠️ +237ms
buildDurationCached	6.9s	6s	N/A
nodeModulesSize	175 MB	175 MB	⚠️ +4.4 kB
nextStartRea..uration (ms)	414ms	415ms	N/A

Client Bundles (main, webpack)

	vercel/next.js canary	vercel/next.js feedthejim/enable-build-worker	Change
199-HASH.js gzip	32.4 kB	32.4 kB	N/A
3f784ff6-HASH.js gzip	53.2 kB	53.2 kB	N/A
99.HASH.js gzip	182 B	182 B	✓
framework-HASH.js gzip	45.5 kB	45.5 kB	✓
main-app-HASH.js gzip	253 B	252 B	N/A
main-HASH.js gzip	35.3 kB	35.4 kB	N/A
webpack-HASH.js gzip	1.75 kB	1.75 kB	N/A
Overall change	45.7 kB	45.7 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	vercel/next.js feedthejim/enable-build-worker	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	vercel/next.js feedthejim/enable-build-worker	Change
_app-HASH.js gzip	206 B	205 B	N/A
_error-HASH.js gzip	182 B	180 B	N/A
amp-HASH.js gzip	506 B	505 B	N/A
css-HASH.js gzip	322 B	323 B	N/A
dynamic-HASH.js gzip	2.59 kB	2.59 kB	✓
edge-ssr-HASH.js gzip	260 B	259 B	N/A
head-HASH.js gzip	350 B	350 B	✓
hooks-HASH.js gzip	369 B	369 B	✓
image-HASH.js gzip	4.38 kB	4.38 kB	N/A
index-HASH.js gzip	256 B	256 B	✓
link-HASH.js gzip	2.67 kB	2.67 kB	N/A
routerDirect..HASH.js gzip	316 B	318 B	N/A
script-HASH.js gzip	385 B	384 B	N/A
withRouter-HASH.js gzip	319 B	319 B	✓
1afbb74e6ecf..834.css gzip	106 B	106 B	✓
Overall change	3.99 kB	3.99 kB	✓

Client Build Manifests

	vercel/next.js canary	vercel/next.js feedthejim/enable-build-worker	Change
_buildManifest.js gzip	484 B	482 B	N/A
Overall change	0 B	0 B	✓

Rendered Page Sizes

	vercel/next.js canary	vercel/next.js feedthejim/enable-build-worker	Change
index.html gzip	528 B	529 B	N/A
link.html gzip	542 B	542 B	✓
withRouter.html gzip	524 B	524 B	✓
Overall change	1.07 kB	1.07 kB	✓

Edge SSR bundle Size

	vercel/next.js canary	vercel/next.js feedthejim/enable-build-worker	Change
edge-ssr.js gzip	96 kB	96 kB	N/A
page.js gzip	140 kB	140 kB	N/A
Overall change	0 B	0 B	✓

Middleware size

	vercel/next.js canary	vercel/next.js feedthejim/enable-build-worker	Change
middleware-b..fest.js gzip	624 B	621 B	N/A
middleware-r..fest.js gzip	150 B	151 B	N/A
middleware.js gzip	25.8 kB	25.8 kB	N/A
edge-runtime..pack.js gzip	1.92 kB	1.92 kB	✓
Overall change	1.92 kB	1.92 kB	✓

Commit: e840807

discussing internally