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/gamepad_button.zig

@@ -0,0 +1,37 @@
+const c = @import("c.zig").c;
+
+/// Gamepad buttons.
+///
+/// See glfw.getGamepadState for how these are used.
+pub const GamepadButton = enum(c_int) {
+    a = c.GLFW_GAMEPAD_BUTTON_A,
+    b = c.GLFW_GAMEPAD_BUTTON_B,
+    x = c.GLFW_GAMEPAD_BUTTON_X,
+    y = c.GLFW_GAMEPAD_BUTTON_Y,
+    left_bumper = c.GLFW_GAMEPAD_BUTTON_LEFT_BUMPER,
+    right_bumper = c.GLFW_GAMEPAD_BUTTON_RIGHT_BUMPER,
+    back = c.GLFW_GAMEPAD_BUTTON_BACK,
+    start = c.GLFW_GAMEPAD_BUTTON_START,
+    guide = c.GLFW_GAMEPAD_BUTTON_GUIDE,
+    left_thumb = c.GLFW_GAMEPAD_BUTTON_LEFT_THUMB,
+    right_thumb = c.GLFW_GAMEPAD_BUTTON_RIGHT_THUMB,
+    dpad_up = c.GLFW_GAMEPAD_BUTTON_DPAD_UP,
+    dpad_right = c.GLFW_GAMEPAD_BUTTON_DPAD_RIGHT,
+    dpad_down = c.GLFW_GAMEPAD_BUTTON_DPAD_DOWN,
+    dpad_left = c.GLFW_GAMEPAD_BUTTON_DPAD_LEFT,
+};
+
+/// Not in the GamepadAxis enumeration as it is a duplicate value which is forbidden.
+pub const last = GamepadButton.dpad_left;
+
+/// Not in the GamepadAxis enumeration as it is a duplicate value which is forbidden.
+pub const cross = GamepadButton.a;
+
+/// Not in the GamepadAxis enumeration as it is a duplicate value which is forbidden.
+pub const circle = GamepadButton.b;
+
+/// Not in the GamepadAxis enumeration as it is a duplicate value which is forbidden.
+pub const square = GamepadButton.x;
+
+/// Not in the GamepadAxis enumeration as it is a duplicate value which is forbidden.
+pub const triangle = GamepadButton.y;