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/glfw/src/mouse_button.zig

@@ -0,0 +1,23 @@
+const c = @import("c.zig").c;
+
+/// Mouse button IDs.
+///
+/// See glfw.setMouseButtonCallback for how these are used.
+pub const MouseButton = enum(c_int) {
+    // We use left/right/middle aliases here because those are more common and we cannot have
+    // duplicate values in a Zig enum.
+    left = c.GLFW_MOUSE_BUTTON_1,
+    right = c.GLFW_MOUSE_BUTTON_2,
+    middle = c.GLFW_MOUSE_BUTTON_3,
+    four = c.GLFW_MOUSE_BUTTON_4,
+    five = c.GLFW_MOUSE_BUTTON_5,
+    six = c.GLFW_MOUSE_BUTTON_6,
+    seven = c.GLFW_MOUSE_BUTTON_7,
+    eight = c.GLFW_MOUSE_BUTTON_8,
+};
+
+/// Not in the MouseButton enumeration as it is a duplicate value which is forbidden.
+pub const last = MouseButton.eight;
+pub const one = MouseButton.left;
+pub const two = MouseButton.right;
+pub const three = MouseButton.middle;