Move sharp to optionalDependencies

[styfle]

This moves sharp to optionalDependencies which prevents the user (in most cases) from needing to manually install sharp like they do with react, for example yarn add next react react-dom.

We originally thought it best to make it a peerDependency and use yarn add next react react-dom sharp however, this would expose implementation details and makes it harder to swap sharp for something else in the future.

The reason for not using dependencies is that sharp is more likely to fail during installation because it relies of native binaries or building from source, and we don't want to block someone from using Next.js just because they can't use Image Optimization.

Lastly, some users might choose to ignore Image Optimization and they can use yarn install --ignore-optional or npm install --no-optional.

Closes #17981

[ijjk]

Stats from current PR

Default Server Mode (Increase detected ⚠️)

General Overall increase ⚠️

	vercel/next.js canary	vercel/next.js sharp-optional-dep	Change
buildDuration	12.8s	12.8s	⚠️ +20ms
nodeModulesSize	62.4 MB	91.6 MB	⚠️ +29.2 MB

Page Load Tests Overall increase ✓

	vercel/next.js canary	vercel/next.js sharp-optional-dep	Change
/ failed reqs	0	0	✓
/ total time (seconds)	2.373	2.341	-0.03
/ avg req/sec	1053.73	1067.94	+14.21
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.298	1.223	-0.07
/error-in-render avg req/sec	1925.76	2043.41	+117.65

Client Bundles (main, webpack, commons)

	vercel/next.js canary	vercel/next.js sharp-optional-dep	Change
677f882d2ed8..35e7.js gzip	11.1 kB	11.1 kB	✓
framework.HASH.js gzip	39 kB	39 kB	✓
main-0297b89..3d55.js gzip	7.34 kB	7.34 kB	✓
webpack-e067..f178.js gzip	751 B	751 B	✓
Overall change	58.1 kB	58.1 kB	✓

Client Bundles (main, webpack, commons) Modern

	vercel/next.js canary	vercel/next.js sharp-optional-dep	Change
677f882d2ed8..dule.js gzip	6.94 kB	6.94 kB	✓
framework.HA..dule.js gzip	39 kB	39 kB	✓
main-8539e6d..dule.js gzip	6.32 kB	6.32 kB	✓
webpack-07c5..dule.js gzip	751 B	751 B	✓
Overall change	53 kB	53 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	vercel/next.js sharp-optional-dep	Change
polyfills-4b..e242.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	vercel/next.js sharp-optional-dep	Change
_app-9a0b9e1..b37e.js gzip	1.28 kB	1.28 kB	✓
_error-ed1b0..8fbd.js gzip	3.44 kB	3.44 kB	✓
hooks-89731c..c609.js gzip	887 B	887 B	✓
index-17468f..5d83.js gzip	227 B	227 B	✓
link-89ad9e7..25bb.js gzip	1.34 kB	1.34 kB	✓
routerDirect..924c.js gzip	284 B	284 B	✓
withRouter-7..c13d.js gzip	284 B	284 B	✓
Overall change	7.74 kB	7.74 kB	✓

Client Pages Modern

	vercel/next.js canary	vercel/next.js sharp-optional-dep	Change
_app-75d3a82..dule.js gzip	625 B	625 B	✓
_error-4469a..dule.js gzip	2.29 kB	2.29 kB	✓
hooks-cbf13f..dule.js gzip	387 B	387 B	✓
index-b9a643..dule.js gzip	226 B	226 B	✓
link-aeb707b..dule.js gzip	1.29 kB	1.29 kB	✓
routerDirect..dule.js gzip	284 B	284 B	✓
withRouter-f..dule.js gzip	282 B	282 B	✓
Overall change	5.39 kB	5.39 kB	✓

Client Build Manifests

	vercel/next.js canary	vercel/next.js sharp-optional-dep	Change
_buildManifest.js gzip	322 B	322 B	✓
_buildManife..dule.js gzip	329 B	329 B	✓
Overall change	651 B	651 B	✓

Rendered Page Sizes

	vercel/next.js canary	vercel/next.js sharp-optional-dep	Change
index.html gzip	1 kB	1 kB	✓
link.html gzip	1.01 kB	1.01 kB	✓
withRouter.html gzip	996 B	996 B	✓
Overall change	3.01 kB	3.01 kB	✓

---

Serverless Mode (Increase detected ⚠️)

General Overall increase ⚠️

	vercel/next.js canary	vercel/next.js sharp-optional-dep	Change
buildDuration	14s	14.2s	⚠️ +217ms
nodeModulesSize	62.4 MB	91.6 MB	⚠️ +29.2 MB

Client Bundles (main, webpack, commons)

	vercel/next.js canary	vercel/next.js sharp-optional-dep	Change
677f882d2ed8..35e7.js gzip	11.1 kB	11.1 kB	✓
framework.HASH.js gzip	39 kB	39 kB	✓
main-0297b89..3d55.js gzip	7.34 kB	7.34 kB	✓
webpack-e067..f178.js gzip	751 B	751 B	✓
Overall change	58.1 kB	58.1 kB	✓

Client Bundles (main, webpack, commons) Modern

	vercel/next.js canary	vercel/next.js sharp-optional-dep	Change
677f882d2ed8..dule.js gzip	6.94 kB	6.94 kB	✓
framework.HA..dule.js gzip	39 kB	39 kB	✓
main-8539e6d..dule.js gzip	6.32 kB	6.32 kB	✓
webpack-07c5..dule.js gzip	751 B	751 B	✓
Overall change	53 kB	53 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	vercel/next.js sharp-optional-dep	Change
polyfills-4b..e242.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	vercel/next.js sharp-optional-dep	Change
_app-9a0b9e1..b37e.js gzip	1.28 kB	1.28 kB	✓
_error-ed1b0..8fbd.js gzip	3.44 kB	3.44 kB	✓
hooks-89731c..c609.js gzip	887 B	887 B	✓
index-17468f..5d83.js gzip	227 B	227 B	✓
link-89ad9e7..25bb.js gzip	1.34 kB	1.34 kB	✓
routerDirect..924c.js gzip	284 B	284 B	✓
withRouter-7..c13d.js gzip	284 B	284 B	✓
Overall change	7.74 kB	7.74 kB	✓

Client Pages Modern

	vercel/next.js canary	vercel/next.js sharp-optional-dep	Change
_app-75d3a82..dule.js gzip	625 B	625 B	✓
_error-4469a..dule.js gzip	2.29 kB	2.29 kB	✓
hooks-cbf13f..dule.js gzip	387 B	387 B	✓
index-b9a643..dule.js gzip	226 B	226 B	✓
link-aeb707b..dule.js gzip	1.29 kB	1.29 kB	✓
routerDirect..dule.js gzip	284 B	284 B	✓
withRouter-f..dule.js gzip	282 B	282 B	✓
Overall change	5.39 kB	5.39 kB	✓

Client Build Manifests

	vercel/next.js canary	vercel/next.js sharp-optional-dep	Change
_buildManifest.js gzip	322 B	322 B	✓
_buildManife..dule.js gzip	329 B	329 B	✓
Overall change	651 B	651 B	✓

Serverless bundles

	vercel/next.js canary	vercel/next.js sharp-optional-dep	Change
_error.js	1.06 MB	1.06 MB	✓
404.html	4.34 kB	4.34 kB	✓
hooks.html	3.92 kB	3.92 kB	✓
index.js	1.06 MB	1.06 MB	✓
link.js	1.1 MB	1.1 MB	✓
routerDirect.js	1.1 MB	1.1 MB	✓
withRouter.js	1.1 MB	1.1 MB	✓
Overall change	5.42 MB	5.42 MB	✓

Commit: 59b59dd