examples: Add select_group_async.zig to trigger the IO groupAsync callback

examples/select_group_async.zig

@@ -0,0 +1,36 @@
+const std = @import("std");
+const Io = std.Io;
+
+const Simulator = @import("zigmulator");
+
+const SelectResult = union(enum) {
+    answer: u32,
+};
+
+fn computeAnswer() u32 {
+    return 42;
+}
+
+fn program(init: std.process.Init) anyerror!void {
+    var buffer: [1]SelectResult = undefined;
+    var select = Io.Select(SelectResult).init(init.io, &buffer);
+
+    // This calls the Io vtable's groupAsync callback.
+    select.async(.answer, computeAnswer, .{});
+
+    const result = try select.await();
+    var stdout = Io.File.stdout().writerStreaming(init.io, &.{});
+    try stdout.interface.print("answer: {}\n", .{result.answer});
+    try stdout.interface.flush();
+}
+
+pub fn main(init: std.process.Init) !void {
+    var sim: Simulator = undefined;
+    sim.init(std.heap.page_allocator, init.io);
+    defer sim.deinit();
+
+    try sim.addExecutable("select_group_async", program);
+    try sim.spawn("select_group_async", .{});
+
+    while (sim.scheduleOne()) {}
+}