Negative variables are not compiled after update

Nobody has claimed this yet.

Did something change regarding the compilation of negative variables?

In v0.14.0 this:

```css
.foo {
--width: 100px;
with: -var(--width);
}
```

used to output:

```css
.foo {
with: -100px;
}
```

But after updating to v0.17.0 I get:

```css
.foo {
with: -var(--width);
}
```

I tried reproducing in the [playground](https://madlittlemods.github.io/postcss-css-variables/playground/), but this still runs on v0.14.0.

Start by comparing the v0.14.0 and v0.17.0 handling of the negative variable example, since the issue provides the expected and actual CSS output and identifies the playground as an entry point. Locate the compilation path for negative variables, add a regression test for the shown input, and verify that the output contains the resolved negative value.

Written by the indexing model from the issue text.