add asy file to generate single card for testing

single-card.asy

@@ -0,0 +1,14 @@
+import "cards.asy" as cards;
+import "numerals.asy" as numerals;
+
+int suit = WHEELS;
+int value = 1;
+filldraw(card, white, black);
+
+if (suit == CARDS && value == 1) {
+  // due to exponential growth, with the larger suit icon on the 1
+  // card, the endpoint of the recursion is a *lot* more visible
+  fill(recursive_card(value, suit, 6), evenodd+red);
+} else {
+  fill(recursive_card(value, suit), evenodd+red);
+}