Add native aspect ratio support

[reinink]

This PR adds native aspect ratio support. Once Safari 15 is popularized, this will have full support in modern browsers.

By default, we include these utilities:

Class	CSS
aspect-auto	aspect-ratio: auto
aspect-square	aspect-ratio: 1 / 1
aspect-video	aspect-ratio: 16 / 9

We considered adding a bunch more by default, but in practice it's easier to just use arbitrary values instead of providing an exhaustive list of numeric values in your config file. For example:

<img class="aspect-[16/9] ... "/>

This utility can be customized under the aspectRatio property in your tailwind.config.js file.

This addition does not conflict with the existing aspect ratio plugin, as it uses different utility names.

[say8425]

When you plan to release this feature?

[reinink]

@say8425 The plan is to release a v3.0 alpha of Tailwind CSS in the next couple of weeks. You can, however, also use an insiders release if you want it today.

[ChazUK]

I have tailwindcss version ^3.1.6 and I'm trying to do class="aspect-video" but no styling is being applied. Is it not yet available in 3.1.6? because the docs say it is https://tailwindcss.com/docs/aspect-ratio