Add support for calc() in arbitrary values

[bradlc]

This PR adds support for calc() within arbitrary values:

<div class="w-[calc(100%+1rem)]"></div>
<div class="w-[calc(-200px/-100px)]"></div>

/* Output */
.w-\[calc\(100\%\+1rem\)\] {
  width: calc(100% + 1rem);
}
.w-\[calc\(-200px\/-10px\)\] {
  width: calc(-200px / -10px);
}