* Split TypeScript Flag Docs

* reword

* Pass config.experimental.cpus to export during build

Currently, there is no way of specifying the number of worker threads of
`next export` when run as part of `next build`.

I suggest a sane default should be to just use the same amount of
workers that were used during the build process which currently seems to
be configured through `config.experimental.cpus`.

This setting is already respected in the two other places where
jest-workers are in use: The TerserPlugin and the staticCheckWorkers in
`next build`.

* Only enable worker threads if there is more than 1 worker

Multiple worker threads can cause problems when certain dependencies are
being used, see e.g. https://github.com/zeit/next.js/issues/7894

This patch allows disabling of worker threads by setting
`config.experimental.cpus = 1`.

The benefit of spawning 1 worker thread, if there is any at all, should
very limited anyways, so the workload can just as well be processed in
the main thread.

* Disable parallel build for firebase authentication example

* Add integration test to cover #7894

* Rename test suite and add worker_threads config

* Disable worker_threads by default

* Update index.test.js

* Use workerThreads config for TerserPlugin

* Update to use workerThreads config in
TerserPlugin for consistency

* Disable node 12 specific test

* Add a configuration flag to disable integrated type checker

* Add tests for typescript-transpileonly

* Restore removed argument

* Make output more coherent

* Split transpileOnly into ignoreDevErrors and ignoreBuildErrors

* Minor stylistic change

* Add warning for getStaticParams without getStaticProps

* Throw error instead of logging to make sure it's noticed

* Lower default concurrency for tests

* Take multiple values into account

* make typescript happy

* allow NextScript to optionally defer javascript

* move defer options to experimental feature

* combine defer flags into a single option

* Update deferScripts to work with serverless target

* Add test for defer and async property

* Read the async property

* Check versions of chrome and chromedriver

* Update to chromedriver 76

* Fix test

* Update Babel dependencies

* Update sizes

Before this patch users would see the following warning during initial
compilation:

```
Warning: Each child in a list should have a unique "key" prop. See https://fb.me/react-warning-keys for more information.
    in link
    in Head
    in html
    in Html
    in Document
    in Context.Provider
    in Context.Provider
```

* Add StrictMode toggle

* Rename option to reactStrictMode

* Default to false initially

* Ensure directory before flushing cache iSSG file

* Add test for prerender cache flush

* Nest the dynamic route test one more level

* update fetch for test

* Update error check

The correct option to use with `micro-cors` package is `allowMethods` instead `allowedMethods`. This PR fixes the example.

* Add tracking src dir usage to telemetry

* Move isSrcDir back to eventVersion

* Move spinner back

* Add test for isSrcDir telemetry

* Add test for dev mode

* Add yarn check to tests

* Update yarn.lock

* Update lockfile some more

* Use resolutions to bump browserslist

* Cache No-Revalidate Pages

* Specifically check for `false`

* Upgrade Lockfile

* Fix types version mismatch

* Upgrade amphtml-validator

* Revert "Upgrade amphtml-validator"

This reverts commit 05068b2ad9f2f53429cf514b47b4f7924b0b9623.

* Disable AMP Tests

* Skip 2x more AMP tests

* Upgrade React dependencies

* Disable data tests

* Increase allowed default sizes

* Warn on No Cache in CI

* Bold the note lead

* Add custom clarification for netlify

* Reorder

* Add tests

* Fix detection

* allow to remove amphtml from desktop page

* only 1 link with amphtml rel

* clean up

* remove checkup for amphtml link

* Dedupe looping

* Register page export declaration to scope

* Update to not need to traverse to update scope

* Dedupe meta tag in favor of custom tag defined in _app

* Add test

* Fix comments

* Don't mark page as nextExport if it's SPR

* Add test for fetching prerender data after mount

* Update to return json if data request

* Update sending data

* Add test using serverless bundle directly

* Don't early return if fromExport

* Update _document.tsx to optimize script loading

* Update maxInitialChunks to 15

* Undo change chunk count

* Adjust impl a bit

* Add CSS preload test

* Ensure build manifest not preloaded

* Correct file name

* break comment

* Add failing tests for SPR

* Fix SPR always revalidating in production

* Remove extra changes

This pull request is a temporary addition that uses the `x-now-route-params` header in serverless.

This header returns the regex groups with indexes, not their named variants.
As a result, we must use the getRouteMatcher utility to reverse this into Next.js' expected names.

Since this got complex, I've added a test for it. We should probably remove this behavior sooner than later.