[FIX] CSS var assignment only for less to less vars

[petermuessig]

In case of more complex expressions as value of a rule, the css variable mapping logic detects this also as simple variable assignment. Therefore we need to introduce a more complex check for a simple variable assignment in the less code:

/resources/sap/ui/core/themes/sap_fiori_3/css-variables.css

SOURCE:
@sapUiShadowHeader: 0 0 0.25rem 0 fade(@sapUiContentShadowColor, 15), inset 0 -0.0625rem 0 0 @sapUiObjectHeaderBorderColor;

WAS (before var assignment feature):
--sapUiShadowHeader:0 0 .25rem 0 rgba(0,0,0,0.15),inset 0 -0.0625rem 0 0 #d9d9d9;

IS (with that var assignment feature):
--sapUiShadowHeader:var(--sapUiObjectHeaderBorderColor);

BECOMES (with this fix here):
--sapUiShadowHeader:0 0 .25rem 0 rgba(0,0,0,0.15),inset 0 -0.0625rem 0 0 #d9d9d9;

[petermuessig]

I also included a test for this scenario.

[petermuessig]

Thanks @matz3