Consider variants starting with @- to be invalid (e.g. @-2xl:flex)

[RobinMalfait]

This PR fixes a small parsing issue where variants such as @-2xl:flex would parse, but were handled as-if they were @2xl:flex instead.

Noticed this while working on: #18867

This is because when we parse normal variants like data-foo then we want to have a data root and a foo value, not a -foo value.

If you are now using @-2xl:flex, then no CSS will be generated for this anymore. If you were relying on this for some reason, you should use @2xl:flex instead.

Test plan

Before:

After:

[mariansimecek]

@RobinMalfait
Hi, this change breaks some styles on our website. I found out that the old migration script automatically generated @-4xl (the current version doesn’t do that). It took me some time to find the root cause of the issue. I just wanted to let you know that there may be people who used the migration script, and now their websites will break. You may want to consider marking this change as breaking.