Fix missing space around - when using % regression

[RobinMalfait]

This PR fixes a regression we shipped in v4.1.9, when using arbitrary values and injecting spaces around operator.

When you use w-[calc(100%-var(--foo))], you expect that this generates valid CSS:

width: calc(100% - var(--foo));

But due to a regression, we generated:

width: calc(100%-var(--foo));

Which is invalid CSS.

This is because the algorithm we used to know when we had to inject a space around the - didn't take the % sign into account.

We also didn't handle uppercase units like 123PX properly. This PR fixes both issues.

Test plan

1. Added a regression test for the %
2. Added a regression test for uppercase units like 123PX

Fixes: #18288