v. 1.0.4, add vh,vw measurements

Took 47 seconds

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

@@ -0,0 +1,31 @@
+package nl.astraeus.css.properties
+
+fun calc(expression: CalcExpression) = Calc(expression)
+
+infix operator fun CalcExpression.plus(other: CalcExpression) = CompoundCalcExpression(
+  this,
+  "+",
+  other
+)
+
+infix operator fun CalcExpression.minus(other: CalcExpression) = CompoundCalcExpression(
+  this,
+  "-",
+  other
+)
+
+interface CalcExpression
+
+class CompoundCalcExpression(
+  val left: CalcExpression,
+  val operator: String,
+  val right: CalcExpression
+): CalcExpression {
+  override fun toString(): String {
+    return "$left $operator $right"
+  }
+}
+
+class Calc(
+  expression: CalcExpression
+) : CssProperty(expression.toString())