Fix generation of utilities that use slashes in arbitrary modifiers

[thecrypticace]

In utilities like text-lg you can specify a line-height / leading value by suffixing it with a config value or an arbitrary value.

For example:

<div class="text-lg/4"></div>
<div class="text-lg/[4rem]"></div>
<div class="text-lg/[calc(4rem+12px)]"></div>

However, you couldn't use a / inside the arbitrary value like so:

<div class="text-lg/[calc(6rem/5)]"></div>

This is because we were looking for the / to split the utility at and we were finding the last / (that's next to the 6rem). Now we skip over the last / because it's inside an arbitrary value which makes sure this utility generates as expected.

Fixes #12362