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:
```css
width: calc(100% - var(--foo));
```
But due to a regression, we generated:
```css
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
ddb0beff66