This PR adds some improvements to the upgrade tool where it can now also
migrate negative arbitrary values to negative bare values.
We already had support for the positive version of this:
```diff
- mb-[32rem]
+ mb-128
```
But now it can also handle negative values:
```diff
- mb-[-32rem]
+ -mb-128
```
The only tricky part here is that we had to hoist the `-` sign. Before
this PR, we were actually generating `mb--128` and that is invalid so it
was thrown out.
## Test plan
1. Added a test to ensure that the negative values are correctly
transformed.
b3fde17eac