Only compile arbitrary values ending in ]

[RobinMalfait]

This PR ensures that if you are using an arbitrary value such as bg-[red that it only compiles if it's a properly closed arbitrary value.

Currently what happens is that it assumes the ] is there, and cuts it off. This then results in the bg-[red to be compiled as:

.bg-\[red {
  background-color: re;
}

Note how the d in red is cut off. That's the assumption that the ] is there.

This PR fixes that by ensuring that the arbitrary value is properly closed.

Fixes: #15484