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/.github/workflows/ci.yml

@@ -0,0 +1,46 @@
+name: CI
+on:
+  - push
+  - pull_request
+jobs:
+  x86_64-linux:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Setup Zig
+        run: |
+          sudo apt install xz-utils
+          sudo sh -c 'wget -c https://ziglang.org/builds/zig-linux-x86_64-0.10.0-dev.3551+92568a009.tar.xz -O - | tar -xJ --strip-components=1 -C /usr/local/bin'
+      - name: x86_64-linux -> aarch64-macos
+        run: zig build test -Dtarget=aarch64-macos.12-none
+      - name: test
+        run: |
+          zig build test
+  x86_64-windows:
+    runs-on: windows-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Install Git
+        run: choco install git
+      - name: Setup Zig
+        run: |
+          $ProgressPreference = 'SilentlyContinue'
+          Invoke-WebRequest -Uri "https://ziglang.org/builds/zig-windows-x86_64-0.10.0-dev.3551+92568a009.zip" -OutFile "C:\zig.zip"
+          cd C:\
+          7z x zig.zip
+          Add-Content $env:GITHUB_PATH "C:\zig-windows-x86_64-0.10.0-dev.3551+92568a009\"
+      - name: test
+        run: zig build test
+  x86_64-macos:
+    runs-on: macos-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Setup Zig
+        run: |
+          brew install xz
+          sudo sh -c 'wget -c https://ziglang.org/builds/zig-macos-x86_64-0.10.0-dev.3551+92568a009.tar.xz -O - | tar -xJ --strip-components=1 -C /usr/local/bin'
+      - name: test
+        run: zig build test