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/basisu/src/transcoder/binding.zig

@@ -0,0 +1,38 @@
+pub const BasisFile = opaque {};
+
+pub extern fn basisu_transcoder_init() void;
+
+pub extern fn transcoder_is_format_supported(tex_type: u32, fmt: u32) bool;
+
+pub extern fn transcoder_init([*]const u8, u32) *BasisFile;
+pub extern fn transcoder_deinit(*BasisFile) void;
+pub extern fn transcoder_get_images_count(*BasisFile) u32;
+pub extern fn transcoder_get_levels_count(*BasisFile, image_index: u32) u32;
+pub extern fn transcoder_get_image_level_desc(
+    *BasisFile,
+    image_index: u32,
+    level_index: u32,
+    orig_width: *u32,
+    orig_height: *u32,
+    total_block: *u32,
+) bool;
+pub extern fn transcoder_get_image_transcoded_size(
+    *BasisFile,
+    image_index: u32,
+    level_index: u32,
+    format: u32,
+    size: *u32,
+) bool;
+pub extern fn transcoder_start_transcoding(*BasisFile) bool;
+pub extern fn transcoder_stop_transcoding(*BasisFile) bool;
+pub extern fn transcoder_transcode(
+    *BasisFile,
+    out: [*]const u8,
+    out_size: u32,
+    image_index: u32,
+    level_index: u32,
+    format: u32,
+    decode_flags: u32,
+    output_row_pitch_in_blocks_or_pixels: u32,
+    output_rows_in_pixels: u32,
+) bool;