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/.github/workflows/m1_ci.yml

@@ -0,0 +1,39 @@
+name: M1
+on:
+  # SECURITY: This must be a push event only, otherwise our M1 mac runner would be compromised by
+  # third-party pull requests which could run arbitrary code. This way, we can restrict it to
+  # collaborators of the repository only.
+  #
+  # Whenever GitHub Actions gets official M1 support, we can use that for PRs and get rid of this
+  # self-hosted runner: https://github.com/actions/virtual-environments/issues/2187 
+  - push
+jobs:
+  aarch64-macos:
+    if: ${{ github.repository == 'hexops/mach-glfw' }}
+    runs-on: [self-hosted, macOS, ARM64]
+    defaults:
+      run:
+        shell: "/usr/bin/arch -arch arm64e /bin/bash --noprofile --norc -eo pipefail {0}"
+    steps:
+      - name: Clean repository submodules
+        # See https://github.com/actions/checkout/issues/385
+        run: |
+          rm -rf $GITHUB_WORKSPACE && mkdir $GITHUB_WORKSPACE
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Setup Zig
+        run: |
+          sudo rm -rf /usr/local/bin/zig /usr/local/bin/lib/
+          sudo sh -c 'wget -c https://ziglang.org/builds/zig-macos-aarch64-0.10.0-dev.3551+92568a009.tar.xz -O - | tar -xJ --strip-components=1 -C /usr/local/bin'
+      - name: test
+        run: zig build test
+        env:
+          AGREE: true
+      - name: aarch64-macos -> x86_64-macos
+        run: zig build test -Dtarget=x86_64-macos
+        env:
+          AGREE: true
+      - name: aarch64-macos -> x86_64-windows
+        run: zig build test -Dtarget=x86_64-windows
+      - name: aarch64-macos -> x86_64-linux
+        run: zig build test -Dtarget=x86_64-linux