make it clear how to use module system without mach.Core (remove mach.App)

Signed-off-by: Stephen Gutekanst <stephen@hexops.com>

examples/core/custom-entrypoint/main.zig

@@ -1,15 +1,41 @@
+const std = @import("std");
 const mach = @import("mach");
 
-// The global list of Mach modules registered for use in our application.
+// The global list of Mach modules our application may use.
 pub const modules = .{
     mach.Core,
     @import("App.zig"),
 };
 
 pub fn main() !void {
-    // Initialize mach.Core
-    try mach.core.initModule();
+    const allocator = std.heap.c_allocator;
 
-    // Main loop
-    while (try mach.core.tick()) {}
+    // Initialize module system
+    try mach.mods.init(allocator);
+
+    // Schedule .app.start to run.
+    mach.mods.schedule(.app, .start);
+
+    // If desired, it is possible to observe when the app has finished starting by dispatching
+    // systems until the app has started:
+    const stack_space = try allocator.alloc(u8, 8 * 1024 * 1024);
+    try mach.mods.dispatchUntil(stack_space, .mach_core, .started);
+
+    // On some platforms, you can drive the mach.Core main loop yourself - but this isn't
+    // possible on all platforms.
+    if (mach.Core.supports_non_blocking) {
+        mach.Core.non_blocking = true;
+        while (mach.mods.mod.mach_core.state != .exited) {
+            // Execute systems until a frame has been finished.
+            try mach.mods.dispatchUntil(stack_space, .mach_core, .frame_finished);
+        }
+    } else {
+        // On platforms where you cannot control the mach.Core main loop, the .mach_core.start
+        // system your app schedules will block forever and the function call below will NEVER
+        // return (std.process.exit will occur first.)
+        //
+        // In this case we can just dispatch systems until there are no more left to execute, which
+        // conviently works even if you aren't using mach.Core in your program.
+        try mach.mods.dispatch(stack_space, .{});
+    }
 }