[Bug] : mergeLonghand should not apply to CSS variables

[redallen]

Describe the bug
mergeLonghand should not apply to CSS vars since it cannot know what the var expands to.

To Reproduce
Input:

.pf-c-select__toggle::before {
    border-color: var(--pf-c-select__toggle--before--BorderTopColor) var(--pf-c-select__toggle--before--BorderRightColor) var(--pf-c-select__toggle--before--BorderBottomColor) var(--pf-c-select__toggle--before--BorderLeftColor);
    border-style: solid;
    border-width: var(--pf-c-select__toggle--before--BorderWidth);
}

Output:

.pf-c-select__toggle:before{
  border-left:var(--pf-c-select__toggle--before--BorderWidth) solid var(--pf-c-select__toggle--before--BorderLeftColor);
  border-bottom:var(--pf-c-select__toggle--before--BorderWidth) solid var(--pf-c-select__toggle--before--BorderBottomColor);
  border-right:var(--pf-c-select__toggle--before--BorderWidth) solid var(--pf-c-select__toggle--before--BorderRightColor);
  border-top:var(--pf-c-select__toggle--before--BorderWidth) solid var(--pf-c-select__toggle--before--BorderTopColor)
}

Expected behavior
Expected output: Same as input.

cssnano@4.1.10