fix: handle missing declaration state in no-invalid-properties

[xbinaryx]

Prerequisites checklist

• I have read the contributing guidelines.

The rule assumed all Function nodes appear within a CSS declaration value. However, constructs such as @import ... layer(theme) or @supports (color: color(display-p3 1 1 1)) can include Function nodes outside of any declaration context. Since declStack is only populated when entering a Declaration node, it was undefined for these cases — leading to:

TypeError: Cannot read properties of undefined (reading 'functionPartsStack')

What is the purpose of this pull request?

This PR fixes a runtime error in the no-invalid-properties rule that occurred when the rule processed Function nodes outside of CSS declarations.

What changes did you make? (Give an overview)

• Added safe-guard checks in the Function, Function > *:not(Function), and Function:exit visitors to ensure that a declaration state exists before accessing it.
• Added tests

Related Issues

Fixes #289

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

[nzakas]

LGTM. Thanks.