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/gpu-dawn/.github/workflows/publish_release.yml

@@ -0,0 +1,45 @@
+name: Publish release
+on:
+  workflow_run:
+    workflows: ["CI"]
+    types:
+      - completed
+concurrency: 
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+jobs:
+  publish:
+    if: ${{ github.event.workflow_run.conclusion == 'success' }}
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v2
+    - name: Record latest release version
+      id: recorded_release_version
+      # Note: we find the latest non-'update to latest binary release' commit here, which is not
+      # necessarily HEAD because another build could have succeeded before us and HEAD may now refer
+      # to 'update to latest binary release' instead of the commit we were building.
+      # By doing this, we ensure any update we push would refer to the absolute latest binary
+      # release even if *this* CI run is older.
+      run: echo "::set-output name=commit::$(git log --oneline | grep -v 'update to latest binary release' | head -n1 | cut -d " " -f1)"
+    # - name: Publish release
+    #   run: echo "TODO(build-system): gh CLI has no way to mark draft as published?"
+    #   env:
+    #     GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+    - name: Checkout hexops/mach monorepo
+      uses: actions/checkout@v2
+      with:
+        repository: hexops/mach
+        token: ${{ secrets.HEXOPS_MACH_GPU_DAWN_PUSH }}
+    - name: 'gpu/dawn: update hard-coded binary release version'
+      env:
+        RELEASE_COMMIT: ${{steps.recorded_release_version.outputs.commit}}
+      run: |
+        sed -i "/binary_version: \[\]const u8 =/c\    binary_version: []const u8 = \"release-$RELEASE_COMMIT\"," ./gpu-dawn/build.zig
+    - name: 'gpu/dawn: use latest binary release'
+      run: |
+        git config user.name 'Release automation'
+        git config user.email 'stephen@hexops.com'
+        git add .
+        git commit -m 'gpu-dawn: update to latest binary release'
+        git push -u origin HEAD