Improve addVariant API (#5809)

* fix incorrect comment

Probably messed this up in another PR, so just a bit of cleaning.

* implement a formatVariantSelector function

This will be used to eventually simplify the addVariant API.

The idea is that it can take a list of strings that define a certain
format. Then it squashes everything to a single format how you would
expect it.

E.g.:

Input:
  - '&:hover'
  - '&:focus'
  - '.dark &'
  - ':merge(.group):hover &'
  - ':merge(.group):focus &'
Output:
  - ':merge(.group):focus:hover .dark &:focus:hover'

The API here is:
  - `&`, this means "The parent" or "The previous selector" (you can
    think of it like if you are using nested selectors)
  - `:merge(.group)`, this means insert a `.group` if it doesn't exist
    yet, but if it does exist already, then merge the new value with the
    old value. This allows us to merge group-focus, group-hover into a
    single `.group:focus:hover ...`

* add new `format`, `withRule` and `wrap` API for addVariant

* implement backwards compatibility

This will ensure that the backwards compatibility for `modifySelectors`
and direct mutations to the `container` will still work.

We will try to capture the changes made to the `rule.selector`, we will
also "backup" the existing selector. This allows us to diff the old and
new selectors and determine what actually happened.

Once we know this, we can restore the selector to the "old" selector and
add the diffed string e.g.: `.foo &`, to the `collectedFormats` as if
you called `format()` directly. This is a bunch of extra work, but it
allows us to be backwards compatible.

In the future we could also warn if you are using `modifySelectors`, but
it is going to be a little bit tricky, because usually that's
implemented by plugin authors and therefore you don't have direct
control over this. Maybe we can figure out the plugin this is used in
and change the warning somehow?

* fix incorrect test

This was clearly a bug, keyframes should not include escaped variants at
all. The reason this is here in the first place is because the nodes in
a keyframe are also "rule" nodes.

* swap the order of pseudo states

The current implementation had a strange side effect, that resulted in
incorrect class definitions. When you are combining the `:hover` and
`:focus` event, then there is no difference between `:hover:focus` and
`:focus:hover`.

However, when you use `:hover::file-selector-button` or `::file-selector-button:hover`,
then there is a big difference. In the first place, you can hover over the full file input
to apply changes to the `File selector button`.
In the second scenario you have to hover over the `File selector button` itself to apply changes.

You can think of it as function calls:
- focus(hover(text-center))

What you would expect is something like this:
`.focus\:hover\:text-center:hover:focus`, where `hover` is on the
inside, and `focus` is on the outside. However in the current
implementation this is implemented as
`.focus\:hover\:text-cener:focus:hover`

* add more variant tests for the new API

* update parallel variants tests to make use of new API

* implement core variants with new API

* simplify/cleanup existing plugin utils

We can get rid of this because we drastically simplified the new
addVariant API.

* add addVariant shorthand signature

The current API looks like this:

```js
addVariant('name', ({ format, wrap }) => {
  // Wrap in an atRule
  wrap(postcss.atRule({ name: 'media', params: '(prefers-reduced-motion: reduce)' }))

  // "Mutate" the selector, for example prepend `.dark`
  format('.dark &')
})
```

It is also pretty common to have this:
```js
addVariant('name', ({ format }) => format('.dark &'))
```
So we simplified this to:
```js
addVariant('name', '.dark &')
```

It is also pretty common to have this:
```js
addVariant('name', ({ wrap }) => wrap(postcss.atRule({ name: 'media', params: '(prefers-reduced-motion: reduce)' })))
```
So we simplified this to:
```js
addVariant('name', '@media (prefers-reduced-motion: reduce)')
```

* improve fontVariantNumeric implementation

We will use `@defaults`, so that only the resets are injected for the
utilities we actually use.

* fix typo

* allow for nested addVariant shorthand

This will allow to write something like:

```js
addVariant('name', `
  @supports (hover: hover) {
    @media (print) {
      &:hover
    }
  }
`)
// Or as a one-liner
addVariant('name', '@supports (hover: hover) { @media (print) { &:hover } }')
```

* update changelog

Author: RobinMalfait

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Changed
 
 - Don't use pointer cursor on disabled buttons by default ([#5772](https://github.com/tailwindlabs/tailwindcss/pull/5772))
+- Improve `addVariant` API ([#5809](https://github.com/tailwindlabs/tailwindcss/pull/5809))
 
 ### Added