Fixes https://github.com/zeit/next.js/issues/12935

Added a few extra types here and there while I was digging through the code

This continues https://github.com/zeit/next.js/pull/12501 and resolves the merge conflicts since we aren't able to update that branch and would like to land this change

Closes: https://github.com/zeit/next.js/pull/12501

Failing test case for https://github.com/zeit/next.js/issues/10488#issuecomment-584500081

This used to give a 500 in dev environment

* Add URL escaping for custom headers

* Update to handle more header values
Co-authored-by: NJJ Kasper <jj@jjsweb.site>

* Add support for params in header key/values

* Update todo

* Update with handling for named patterns

* Use compiled package
Co-authored-by: NJoe Haddad <joe.haddad@zeit.co>

* Add basePath in link component and add/remove it consistently

* Update to not use regex for delBasePath

* Expose addBasePath as router method

* Revert "Expose addBasePath as router method"

This reverts commit 40fed596195c6affabf837e42d472452768e13a3.

* Expose basePath as router field

* Apply suggestion

* Expose basePath as router field

* remove un-used vars

* Update externals

* Apply lint fix

* Update size-limit test

* Update prefetch

* Add error/warning when a page is rendered without a /

Throws an error for development and gives a warning in production

* Add tests for error when rendering without starting slash

* Update to always warn and add err.sh

* Update errors/render-no-starting-slash.md
Co-authored-by: NJJ Kasper <jj@jjsweb.site>
Co-authored-by: NTim Neutkens <tim@timneutkens.nl>

* Warn when user has pages/_error but no pages/404

* Update test

* Update custom-error-no-custom-404.md

* Apply suggestions from code review
Co-Authored-By: NTim Neutkens <tim@timneutkens.nl>
Co-authored-by: NTim Neutkens <tim@timneutkens.nl>

* Remove req.env

* Rename NEXT_APP_ to NEXT_PUBLIC_

* Remove key exposing env config

* Update tests

* Update NEXT_APP_ test to NEXT_PUBLIC_

* Test `static/` file name encoding

* Fix `static/` file name encoding

* Add initial support for new env config file

* Fix serverless processEnv call when no env is provided

* Add missing await for test method

* Update env config to .env.json and add dotenv loading

* ncc dotenv package

* Update type

* Update with new discussed behavior removing .env.json

* Update hot-reloader createEntrypoints

* Make sure .env is loaded before next.config.js

* Add tests for all separate .env files

* Remove comments

* Add override tests

* Add test for overriding env vars based on local environment

* Add support for .env.test

* Apply suggestions from code review
Co-Authored-By: NJoe Haddad <joe.haddad@zeit.co>

* Use chalk for env loaded message

* Remove constant as it’s not needed

* Update test

* Update errsh, taskr, and CNA template ignores

* Make sure to only consider undefined missing

* Remove old .env ignore

* Update to not populate process.env with loaded env

* Add experimental flag and add loading of global env values
Co-authored-by: NTim Neutkens <timneutkens@me.com>
Co-authored-by: NJoe Haddad <joe.haddad@zeit.co>

* Add custom amp optimizer and skip validation mode

* fix: type

* fix worker lint errors

* Ensure hybrid AMP works correctly with SSG

* Strip AMP from query when not needed

* Correct Cache-Control Behavior for GS(S)P

* remove old line

* fix test

* Update handling for unnamed params and named patterns in custom-routes

* Update query handling to match Now

* Verify GS(S)P Serializability

* Add support for cyclic refs

* Add unit tests

* Test for error in development mode

* Fix prerender preview tests

* Fix gssp preview tests

* fix 2x test cases

* Add desired test

* fix some more tests

* Fix route manifest expect

* Fix test expects

* Test that `getServerSideProps` does not error in production

* Test that getStaticProps is not checked in production

* Test serialization check during build

* Fix export detection for serverless

* Update test/unit/is-serializable-props.test.js
Co-Authored-By: NJJ Kasper <jj@jjsweb.site>
Co-authored-by: NJJ Kasper <jj@jjsweb.site>

* Update to make sure preview mode works with getServerSideProps

* Update to only parse previewData in GS(S)P mode

* Throw a NoFallbackError instead of returning

* Replace invariant
Co-authored-by: NJJ Kasper <jj@jjsweb.site>

* Correctly pass preview data

* remove todo

* re-do change

* fix types

* Prevent regression

* Remove Dead Code from Next Server

* remove options

* trigger

* Extract getStaticPaths helper

* Extract sendPayload and prepareServerlessUrl

* Remove erroneously included file