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/main.zig

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+// Experimental ECS app example. Not yet ready for actual use.
+
+const std = @import("std");
+const mach = @import("mach");
+const gpu = mach.gpu;
+const ecs = mach.ecs;
+
+// TODO: rename *ecs.World to *engine.Engine or something
+
+const renderer = @import("renderer.zig");
+const physics2d = @import("physics2d.zig");
+
+// Define all the modules in our application. Modules can have components, systems, state,
+// and/or global values in them. They can also send and receive messages to coordinate
+// with each-other.
+//
+// Single-word module names (`.mach`, `.renderer`, etc.) are reserved for the application itself.
+//
+// Modules that come from libraries must be prefixed (e.g. `.bullet_physics`, `.ziglibs_box2d`)
+// similar to GitHub repositories, to avoid conflicts with one another. Note that modules themselves
+// will interact with the ECS using e.g. `.getComponent(.bullet_physics, .location)` internally and
+// so cannot be renamed here.
+// TODO: just make this a list so one cannot even think renaming is possible here
+const modules = ecs.Modules(.{
+    .mach = mach.module,
+    .renderer = renderer.module,
+    .physics2d = physics2d.module,
+});
+
+// Our Mach app, which tells Mach where to find our modules and init entry point.
+pub const App = mach.App(modules, init);
+
+pub fn init(engine: *ecs.World(modules)) !void {
+    // The Mach .core is where we set window options, etc.
+    const core = engine.get(.mach, .core);
+    try core.setOptions(.{ .title = "Hello, ECS!" });
+
+    // We can get the GPU device:
+    const device = engine.get(.mach, .device);
+    _ = device; // TODO: actually show off using the GPU device
+
+    // We can create entities, and set components on them. Note that components live in a module
+    // namespace, so we set the `.renderer, .location` component which is different than the
+    // `.physics2d, .location` component.
+
+    // TODO: cut out the `.entities.` in this API to make it more brief
+    const player = try engine.entities.new();
+    try engine.entities.setComponent(player, .renderer, .location, .{ .x = 0, .y = 0, .z = 0 });
+    try engine.entities.setComponent(player, .physics2d, .location, .{ .x = 0, .y = 0 });
+    _ = player;
+
+    // TODO: there could be an entities wrapper to interact with a single namespace so you don't
+    // have to pass it in as a parameter always?
+}

examples/ecs-app/physics2d.zig

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+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", .{}),
+    }
+}

examples/ecs-app/renderer.zig

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+const mach = @import("mach");
+const ecs = mach.ecs;
+
+pub const module = ecs.Module(.{
+    .components = .{
+        .location = Vec3,
+        .rotation = Vec3,
+    },
+    // TODO: there would be systems that we register here. Functions that iterate over entities
+    // with renderer components like `.geometry` and render them for example!
+});
+
+pub const Vec3 = struct { x: f32, y: f32, z: f32 };