Merge function-builder branch into master

src/commonMain/kotlin/nl/astraeus/css/properties/ZIndex.kt

@@ -0,0 +1,13 @@
+package nl.astraeus.css.properties
+
+class ZIndex(
+    value: String
+) : CssProperty(value) {
+
+    companion object {
+        val auto = ZIndex("auto")
+        val initial = ZIndex("initial")
+        val inherit = ZIndex("inherit")
+    }
+
+}