[CSS Stylelint] Disallow appending unit to zero values

[sgiehl]

Description:

Please include a description of this change and which issue it fixes. If no issue exists yet please include context and what problem it solves.

Review

• Functional review done
• Potential edge cases thought about (behavior of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• Security review done
• Wording review done
• Code review done
• Tests were added if useful/possible
• Reviewed for breaking changes
• Developer changelog updated if needed
• Documentation added if needed
• Existing documentation updated if needed

[bx80]

This makes sense. PHPStorm complains about zero values with units by default, so it's consistent to have it as a lint rule too 👍