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/main.zig

@@ -0,0 +1,45 @@
+pub usingnamespace @import("encoder.zig");
+pub usingnamespace @import("transcoder.zig");
+
+pub const BasisTextureFormat = enum(u1) {
+    etc1s = 0,
+    uastc4x4 = 1,
+};
+
+// Test
+
+const t = @import("transcoder.zig");
+const e = @import("encoder.zig");
+const testing = @import("std").testing;
+
+test "reference decls" {
+    testing.refAllDeclsRecursive(t);
+    testing.refAllDeclsRecursive(e);
+}
+
+test "encode/transcode" {
+    // Encode
+    e.init_encoder();
+
+    const params = e.CompressorParams.init(1);
+    params.setGenerateMipMaps(true);
+    params.setBasisFormat(.uastc4x4);
+    params.setPackUASTCFlags(.{ .fastest = true });
+    defer params.deinit();
+
+    const image = params.getImageSource(0);
+    image.fill(@embedFile("../test/ziggy.png"), 379, 316, 4);
+
+    const comp = try e.Compressor.init(params);
+    try comp.process();
+
+    // Transcode
+    t.init_transcoder();
+
+    const trans = try t.Transcoder.init(comp.output());
+    defer trans.deinit();
+
+    var out_buf = try testing.allocator.alloc(u8, try trans.calcTranscodedSize(0, 0, .astc_4x4_rgba));
+    defer testing.allocator.free(out_buf);
+    try trans.transcode(out_buf, 0, 0, .astc_4x4_rgba, .{});
+}