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/sysjs/build.zig

@@ -0,0 +1,25 @@
+const std = @import("std");
+
+pub fn build(b: *std.build.Builder) void {
+    const mode = b.standardReleaseOptions();
+    const target = b.standardTargetOptions(.{});
+    const test_step = b.step("test", "Run library tests");
+    test_step.dependOn(&testStep(b, mode, target).step);
+}
+
+pub fn testStep(b: *std.build.Builder, mode: std.builtin.Mode, target: std.zig.CrossTarget) *std.build.RunStep {
+    const main_tests = b.addTestExe("sysjs-tests", thisDir() ++ "/src/main.zig");
+    main_tests.setBuildMode(mode);
+    main_tests.setTarget(target);
+    return main_tests.run();
+}
+
+pub const pkg = std.build.Pkg{
+    .name = "sysjs",
+    .source = .{ .path = thisDir() ++ "/src/main.zig" },
+    .dependencies = &[_]std.build.Pkg{},
+};
+
+fn thisDir() []const u8 {
+    return std.fs.path.dirname(@src().file) orelse ".";
+}