add ex086 async 3

exercises/086_async3.zig

@@ -0,0 +1,29 @@
+//
+// Because they can suspend and resume, async Zig functions are
+// an example of a more general programming concept called
+// "coroutines". One of the neat things about Zig async functions
+// is that they retain their state as they are suspended and
+// resumed.
+//
+// See if you can make this program print "5 4 3 2 1".
+//
+const print = @import("std").debug.print;
+
+pub fn main() void {
+    const n = 5;
+    var foo_frame = async foo(n);
+
+    ???
+
+    print("\n", .{});
+}
+
+fn foo(countdown: u32) void {
+    var current = countdown;
+
+    while (current > 0) {
+        print("{} ", .{current});
+        current -= 1;
+        suspend;
+    }
+}