all: move standalone libraries to libs/ subdirectory

The root dir of our repository has grown quite a lot the past few months.

I'd like to make it more clear where the bulk of the engine lives (`src/`) and
also make it more clear which Mach libraries are consumable as standalone projects.

As for the name of this directory, `libs` was my first choice but there's a bit of
a convention of that being external libraries in Zig projects _today_, while these
are libraries maintained as part of Mach in this repository - not external ones.

We will name this directory `libs`, and if we have a need for external libraries
we will use `external` or `deps` for that directory name. I considered other names
such as `components`, `systems`, `modules` (which are bad as they overlap with
major ECS / engine concepts), and it seems likely the official Zig package manager
will break the convention of using a `libs` dir anyway.

Performed via:

```sh
mkdir libs/
git mv freetype libs/
git mv basisu libs/
git mv gamemode libs/
git mv glfw libs/
git mv gpu libs/
git mv gpu-dawn libs/
git mv sysaudio libs/
git mv sysjs libs/
git mv ecs libs/
```

git-subtree-dir: glfw
git-subtree-mainline: 0d5b853443
git-subtree-split: 572d1144f11b353abdb64fff828b25a4f0fbb7ca

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

git mv ecs libs/

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

libs/gpu-dawn/build.zig

@@ -0,0 +1,29 @@
+const std = @import("std");
+const Builder = std.build.Builder;
+const glfw = @import("libs/mach-glfw/build.zig");
+const system_sdk = @import("libs/mach-glfw/system_sdk.zig");
+const gpu_dawn_sdk = @import("sdk.zig");
+
+pub fn build(b: *Builder) void {
+    const mode = b.standardReleaseOptions();
+    const target = b.standardTargetOptions(.{});
+    const gpu_dawn = gpu_dawn_sdk.Sdk(.{
+        .glfw = glfw,
+        .glfw_include_dir = "libs/mach-glfw/upstream/glfw/include",
+        .system_sdk = system_sdk,
+    });
+
+    const options = gpu_dawn.Options{
+        .from_source = b.option(bool, "dawn-from-source", "Build Dawn from source") orelse false,
+    };
+
+    // Just to demonstrate/test linking. This is not a functional example, see the mach/gpu examples
+    // or Dawn C++ examples for functional example code.
+    const example = b.addExecutable("dawn-example", "src/main.zig");
+    example.setBuildMode(mode);
+    example.setTarget(target);
+    gpu_dawn.link(b, example, options);
+    glfw.link(b, example, .{ .system_sdk = .{ .set_sysroot = false } });
+    example.addPackage(glfw.pkg);
+    example.install();
+}