examples: add initial ecs-app

This is a temporary application to begin iterating on high-level ECS applications.

Eventually, this will be removed - for now it's just here so we can see how this API
looks today and improve it.

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

examples/ecs-app/physics2d.zig

@@ -0,0 +1,26 @@
+const mach = @import("mach");
+const ecs = mach.ecs;
+const std = @import("std");
+
+pub const Message = ecs.Messages(.{
+    .tick = void,
+});
+
+pub const module = ecs.Module(.{
+    .components = .{
+        .location = Vec2,
+        .rotation = Vec2,
+        .velocity = Vec2,
+    },
+    .messages = Message,
+    .update = update,
+});
+
+pub const Vec2 = struct { x: f32, y: f32 };
+
+fn update(msg: Message) void {
+    switch (msg) {
+        // TODO: implement queries, ability to set components, etc.
+        .tick => std.debug.print("\nphysics tick!\n", .{}),
+    }
+}