fix: no-invalid-properties false positives for var() in functions

[thecalamiity]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request?

This PR fixes a regression in the no-invalid-properties rule where it incorrectly reports for valid CSS properties containing var() usage within functions like calc().

What changes did you make? (Give an overview)

• Updated the validation logic to properly handle CSS variables when they appear inside functions.
• Added tests to verify correct behavior for variables in functions

Related Issues

Fixes #223

Is there anything you'd like reviewers to focus on?

[nzakas]

Here we're doing a traversal on top of the traversal that is already happening in the rule, which is inefficient.

A more efficient approach would be to use the Function visitor method to do the traversal automatically. Can you take a look at that?

[thecalamiity]

I looked into using the Function visitor method here, but I don’t see a straightforward way to apply it. The code at this point isn’t just traversing—it’s reconstructing the property value as a string, including nested function names and arguments. The visitor pattern alone doesn’t provide the context needed for this reconstruction, so a manual traversal seems necessary. If you know of a way to leverage the visitor pattern for this case, I’d appreciate your input.