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/src/callbacks.zig

@@ -0,0 +1,110 @@
+const CompilationInfoRequestStatus = @import("types.zig").CompilationInfoRequestStatus;
+const CompilationInfo = @import("types.zig").CompilationInfo;
+const ErrorType = @import("types.zig").ErrorType;
+const LoggingType = @import("types.zig").LoggingType;
+const RequestDeviceStatus = @import("types.zig").RequestDeviceStatus;
+const RequestAdapterStatus = @import("types.zig").RequestAdapterStatus;
+const CreatePipelineAsyncStatus = @import("types.zig").CreatePipelineAsyncStatus;
+const Device = @import("device.zig").Device;
+const Adapter = @import("adapter.zig").Adapter;
+const ComputePipeline = @import("compute_pipeline.zig").ComputePipeline;
+const RenderPipeline = @import("render_pipeline.zig").RenderPipeline;
+
+pub const CompilationInfoCallback = if (@import("builtin").zig_backend == .stage1)
+    fn (
+        status: CompilationInfoRequestStatus,
+        compilation_info: *const CompilationInfo,
+        userdata: ?*anyopaque,
+    ) callconv(.C) void
+else
+    *const fn (
+        status: CompilationInfoRequestStatus,
+        compilation_info: *const CompilationInfo,
+        userdata: ?*anyopaque,
+    ) callconv(.C) void;
+
+pub const ErrorCallback = if (@import("builtin").zig_backend == .stage1)
+    fn (
+        typ: ErrorType,
+        message: [*:0]const u8,
+        userdata: ?*anyopaque,
+    ) callconv(.C) void
+else
+    *const fn (
+        typ: ErrorType,
+        message: [*:0]const u8,
+        userdata: ?*anyopaque,
+    ) callconv(.C) void;
+
+pub const LoggingCallback = if (@import("builtin").zig_backend == .stage1)
+    fn (
+        typ: LoggingType,
+        message: [*:0]const u8,
+        userdata: ?*anyopaque,
+    ) callconv(.C) void
+else
+    *const fn (
+        typ: LoggingType,
+        message: [*:0]const u8,
+        userdata: ?*anyopaque,
+    ) callconv(.C) void;
+
+pub const RequestDeviceCallback = if (@import("builtin").zig_backend == .stage1)
+    fn (
+        status: RequestDeviceStatus,
+        device: *Device,
+        message: ?[*:0]const u8,
+        userdata: ?*anyopaque,
+    ) callconv(.C) void
+else
+    *const fn (
+        status: RequestDeviceStatus,
+        device: *Device,
+        message: ?[*:0]const u8,
+        userdata: ?*anyopaque,
+    ) callconv(.C) void;
+
+pub const RequestAdapterCallback = if (@import("builtin").zig_backend == .stage1)
+    fn (
+        status: RequestAdapterStatus,
+        adapter: *Adapter,
+        message: ?[*:0]const u8,
+        userdata: ?*anyopaque,
+    ) callconv(.C) void
+else
+    *const fn (
+        status: RequestAdapterStatus,
+        adapter: *Adapter,
+        message: ?[*:0]const u8,
+        userdata: ?*anyopaque,
+    ) callconv(.C) void;
+
+pub const CreateComputePipelineAsyncCallback = if (@import("builtin").zig_backend == .stage1)
+    fn (
+        status: CreatePipelineAsyncStatus,
+        compute_pipeline: *ComputePipeline,
+        message: [*:0]const u8,
+        userdata: ?*anyopaque,
+    ) callconv(.C) void
+else
+    *const fn (
+        status: CreatePipelineAsyncStatus,
+        compute_pipeline: *ComputePipeline,
+        message: [*:0]const u8,
+        userdata: ?*anyopaque,
+    ) callconv(.C) void;
+
+pub const CreateRenderPipelineAsyncCallback = if (@import("builtin").zig_backend == .stage1)
+    fn (
+        status: CreatePipelineAsyncStatus,
+        pipeline: *RenderPipeline,
+        message: [*:0]const u8,
+        userdata: ?*anyopaque,
+    ) callconv(.C) void
+else
+    *const fn (
+        status: CreatePipelineAsyncStatus,
+        pipeline: *RenderPipeline,
+        message: [*:0]const u8,
+        userdata: ?*anyopaque,
+    ) callconv(.C) void;