Remove AOT (#5340)

* make `jit` mode the default when no mode is specified

* unify JIT and AOT codepaths

* ensure `Object.entries` on undefined doesn't break

It could be that sometimes you don't have values in your config (e.g.: `presets: []`), this in turn will break some plugins where we assume we have a value.

* drop AOT specific tests

These tests are all covered by JIT mode already and were AOT specific.

* simplify tests, and add a few

Some of the tests were written for AOT specifically, some were missing. We also updated the way we write those tests, essentially making Tailwind a blackbox, by testing against the final output.
Now that JIT mode is the default, this is super fast because we only generate what is used, instead of partially testing in a 3MB file or building it all, then purging.

* add some todo's to make sure we warn in a few cases

* make `darkMode: 'media'`, the default

This also includes moving dark mode tests to its own dedicated file.

* remove PostCSS 7 compat mode

* update CLI to be JIT-first

* fix integration tests

This is not a _real_ fix, but it does solve the broken test for now.

* warn when using @responsive or @variants

* remove the JIT preview warning

* remove AOT-only code paths

* remove all `mode: 'jit'` blocks

Also remove `variants: {}` since they are not useful in `JIT` mode
anymore.

* drop unused dependencies

* rename `purge` to `content`

* remove static CDN builds

* mark `--purge` as deprecated in the CLI

This will still work, but a warning will be printed and it won't show up
in the `--help` output.

* cleanup nesting plugin

We don't have to duplicate it anymore since there is no PostCSS 7
version anymore.

* make sure integration tests run in band

* cleanup folder structure

* make sure nesting folder is available

* simplify resolving of purge/content information

tests/fixtures/custom-config.js

@@ -1,4 +1,5 @@
 module.exports = {
+  content: [{ raw: '<div class="mobile:font-bold"></div>' }],
   theme: {
     screens: {
       mobile: '400px',

tests/fixtures/custom-purge-config.js

@@ -1,5 +1,5 @@
 module.exports = {
-  purge: ['./tests/fixtures/*.html'],
+  content: ['./tests/fixtures/*.html'],
   theme: {
     extend: {
       colors: {