add support for esm externals

[sokra]

add experimental.esmExternals: boolean | 'loose' config option

remove output.environment configuration in favor of target

	esmExternals: false (default)	esmExternals: 'loose'	esmExternals: true
import cjs package	require()	require()	require()
require cjs package	require()	require()	require()
import mixed package	require() ***	import()	import()
require mixed package	require()	require()	require()
import pure esm package	import()	import()	import()
require pure esm package	Error **	import() *	Error **
import pure cjs package	require()	require()	Resolving error
require pure cjs package	require()	require()	require()

cjs package: Offers only CJS implementation (may not even have an exports field)
mixed package: Offers CJS and ESM implementation via exports field
pure esm package: Only offers an ESM implementation (may not even have an exports field)
pure cjs package: CommonJs package that prevents importing via exports field when import is used.

* This case will behave a bit unexpected for now, since require will return a Promise. So that need to be awaited. This will be fixed once the whole next.js bundle is ESM. It didn't work at all before this PR.
** This is a new Error when trying to require an esm package.
*** For mixed packages we prefer the CommonJS variant to avoid a breaking change.

Feature

• Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
• Related issues linked using fixes #number
• Integration tests added
• Documentation added
• Telemetry added. In case of a feature if it's used or not.
• Errors have helpful link attached, see contributing.md

Documentation / Examples

• Make sure the linting passes

[ijjk]

Stats from current PR

Default Build (Increase detected ⚠️)

General Overall increase ⚠️

	vercel/next.js canary	vercel/next.js feature/esm-externals	Change
buildDuration	14.9s	14.7s	-220ms
buildDurationCached	3.7s	3.4s	-284ms
nodeModulesSize	49.3 MB	49.3 MB	⚠️ +10.4 kB

Page Load Tests Overall increase ✓

	vercel/next.js canary	vercel/next.js feature/esm-externals	Change
/ failed reqs	0	0	✓
/ total time (seconds)	2.366	2.396	⚠️ +0.03
/ avg req/sec	1056.84	1043.43	⚠️ -13.41
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.344	1.315	-0.03
/error-in-render avg req/sec	1860.52	1900.68	+40.16

Client Bundles (main, webpack, commons)

	vercel/next.js canary	vercel/next.js feature/esm-externals	Change
359.HASH.js gzip	3.09 kB	3.09 kB	✓
framework-HASH.js gzip	42 kB	42 kB	✓
main-HASH.js gzip	20.6 kB	20.6 kB	✓
webpack-HASH.js gzip	1.49 kB	1.49 kB	✓
Overall change	67.2 kB	67.2 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	vercel/next.js feature/esm-externals	Change
polyfills-HASH.js gzip	31.1 kB	31.1 kB	✓
Overall change	31.1 kB	31.1 kB	✓

Client Pages

	vercel/next.js canary	vercel/next.js feature/esm-externals	Change
_app-HASH.js gzip	803 B	803 B	✓
_error-HASH.js gzip	3.18 kB	3.18 kB	✓
amp-HASH.js gzip	526 B	526 B	✓
css-HASH.js gzip	329 B	329 B	✓
hooks-HASH.js gzip	903 B	903 B	✓
image-HASH.js gzip	5.69 kB	5.69 kB	✓
index-HASH.js gzip	261 B	261 B	✓
link-HASH.js gzip	1.66 kB	1.66 kB	✓
routerDirect..HASH.js gzip	319 B	319 B	✓
withRouter-HASH.js gzip	320 B	320 B	✓
bb14e60e810b..30f.css gzip	125 B	125 B	✓
Overall change	14.1 kB	14.1 kB	✓

Client Build Manifests

	vercel/next.js canary	vercel/next.js feature/esm-externals	Change
_buildManifest.js gzip	419 B	419 B	✓
Overall change	419 B	419 B	✓

Rendered Page Sizes

	vercel/next.js canary	vercel/next.js feature/esm-externals	Change
index.html gzip	531 B	531 B	✓
link.html gzip	544 B	544 B	✓
withRouter.html gzip	524 B	524 B	✓
Overall change	1.6 kB	1.6 kB	✓

---

Webpack 4 Mode (Decrease detected ✓)

General Overall increase ⚠️

	vercel/next.js canary	vercel/next.js feature/esm-externals	Change
buildDuration	12s	11.6s	-384ms
buildDurationCached	4.8s	4.7s	-52ms
nodeModulesSize	49.3 MB	49.3 MB	⚠️ +10.4 kB

Page Load Tests Overall decrease ⚠️

	vercel/next.js canary	vercel/next.js feature/esm-externals	Change
/ failed reqs	0	0	✓
/ total time (seconds)	2.429	2.463	⚠️ +0.03
/ avg req/sec	1029.34	1015.18	⚠️ -14.16
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.339	1.337	0
/error-in-render avg req/sec	1867.42	1869.37	+1.95

Client Bundles (main, webpack, commons)

	vercel/next.js canary	vercel/next.js feature/esm-externals	Change
14.HASH.js gzip	3.11 kB	3.11 kB	✓
677f882d2ed8..HASH.js gzip	13.9 kB	13.9 kB	✓
framework.HASH.js gzip	41.8 kB	41.8 kB	✓
main-HASH.js gzip	7.81 kB	7.81 kB	✓
webpack-HASH.js gzip	1.19 kB	1.19 kB	✓
Overall change	67.8 kB	67.8 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	vercel/next.js feature/esm-externals	Change
polyfills-HASH.js gzip	31.3 kB	31.3 kB	✓
Overall change	31.3 kB	31.3 kB	✓

Client Pages

	vercel/next.js canary	vercel/next.js feature/esm-externals	Change
_app-HASH.js gzip	791 B	791 B	✓
_error-HASH.js gzip	3.83 kB	3.83 kB	✓
amp-HASH.js gzip	531 B	531 B	✓
css-HASH.js gzip	333 B	333 B	✓
hooks-HASH.js gzip	910 B	910 B	✓
index-HASH.js gzip	230 B	230 B	✓
link-HASH.js gzip	1.64 kB	1.64 kB	✓
routerDirect..HASH.js gzip	297 B	297 B	✓
withRouter-HASH.js gzip	293 B	293 B	✓
e025d2764813..52f.css gzip	125 B	125 B	✓
Overall change	8.98 kB	8.98 kB	✓

Client Build Manifests

	vercel/next.js canary	vercel/next.js feature/esm-externals	Change
_buildManifest.js gzip	418 B	418 B	✓
Overall change	418 B	418 B	✓

Rendered Page Sizes

	vercel/next.js canary	vercel/next.js feature/esm-externals	Change
index.html gzip	574 B	574 B	✓
link.html gzip	586 B	586 B	✓
withRouter.html gzip	569 B	569 B	✓
Overall change	1.73 kB	1.73 kB	✓

Commit: d3bcbac

[timneutkens]

Looks good as discussed 👍