Add matchVariant API

[philipp-spiess]

This PR adds support for the matchVariant plugin API. I've copied over all V3 tests and made sure they still pass.

Sorted order of stacked arbitrary variants

The only difference in behavior is regarding the sort order of stacked arbitrary variants: Sorting in this case now works by the latest defined matchVariant taking precedence.

So, if you define a plugin like this:

matchVariant('testmin', (value) => `@media (min-width: ${value})`, {
  sort(a, z) {
    return parseInt(a.value) - parseInt(z.value)
  },
})

matchVariant('testmax', (value) => `@media (max-width: ${value})`, {
  sort(a, z) {
    return parseInt(z.value) - parseInt(a.value)
  },
})

The resulting CSS is first sorted by the testmax values descending and then the testmin values ascending, so these candidates:

testmin-[150px]:testmax-[400px]:order-2
testmin-[100px]:testmax-[350px]:order-3
testmin-[100px]:testmax-[300px]:order-4
testmin-[100px]:testmax-[400px]:order-1

Will resolve to the order outlined by the order- utility.

At-rules and placeholders support

Since we added support for at-rules and placeholders in the matchVariant syntax like this:

matchVariant(
  'potato',
  (flavor) => `@media (potato: ${flavor}) { @supports (font:bold) { &:large-potato } }`,
)

We also added support for the same syntax to the addVariant API:

addVariant(
  'potato',
  '@media (max-width: 400px) { @supports (font:bold) { &:large-potato } }',
)

The only change necessary in core was to call functional variants for when the variant value is set to null. This allows functional variants to define the un-parameterized implementation like potato:underline as opposed to potato[big]:underline.