Add line-height modifier for font-size utilities

[adamwathan]

This PR makes use of generalized modifier support added in v3.2 to add a new line-height modifier to the existing font-size utilities.

This makes it possible to define your font-size and line-height together in a single class for terser code, like you can when using the opacity modifier with color utilities.

<!-- Before -->
<div class="text-lg leading-8">

<!-- Now -->
<div class="text-lg/8">

The potential modifier values are pulled from your lineHeight configuration. You can also use arbitrary values like this:

<!-- Arbitrary line-height modifier -->
<div class="text-xl/[30px]">

<!-- Arbitrary font-size with configured line-height modifier -->
<div class="text-[19px]/7">

<!-- Arbitrary font-size with arbitrary line-height modifier -->
<div class="text-[19px]/[27px]">

The leading-* classes of course aren't going anywhere and are still both useful and supported.

Need to prepare documentation before merging.

[danhollick]

@adamwathan just to clarify, is using a relative line-height possible with this modifier?

<!-- Before -->
<div class="text-lg leading-snug">

<!-- Now -->
<div class="text-lg/snug">

Update: it is possible