chore: minor improvements to Size

Author: quintesse

src/main/java/org/codejive/context/terminal/Size.java

@@ -5,6 +5,8 @@ public class Size {
     private final int height;
 
     public Size(int width, int height) {
+        assert width >= 0;
+        assert height >= 0;
         this.width = width;
         this.height = height;
     }
@@ -16,4 +18,9 @@ public int width() {
     public int height() {
         return height;
     }
+
+    @Override
+    public String toString() {
+        return width + "x" + height;
+    }
 }