LibWeb/CSS: Implement the light-dark color function

Author: https://github.com/Gingeh
Commit: 8e56109515
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/3146
Reviewed-by: https://github.com/AtkinsSJ ✅

Tests/LibWeb/Text/expected/wpt-import/css/css-color/light-dark-basic.txt

@@ -0,0 +1,7 @@
+Harness status: OK
+
+Found 2 tests
+
+2 Pass
+Pass	light-dark(white, black)
+Pass	light-dark(light-dark(white, red), red)

Tests/LibWeb/Text/expected/wpt-import/css/css-color/light-dark-currentcolor-in-color.txt

@@ -0,0 +1,6 @@
+Harness status: OK
+
+Found 1 tests
+
+1 Fail
+Fail	curentColor in light-dark() refers to parent color