add workflow to create source tarball

.github/workflows/ci.yml

@@ -17,6 +17,21 @@ env:
   R2V: 5.9.0
 
 jobs:
+  sources:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+    - uses: actions/setup-python@v4
+      with:
+        python-version: 3.7.x
+    - name: apt dependencies
+      run: sudo pip3 install meson ninja
+    - name: fetch dependencies via meson
+      run: meson subprojects download
+    - uses: actions/upload-artifact@v3
+      with:
+        name: r2dec-${{env.R2V}}-src-with-deps.zip
+        path: .
   linux:
     runs-on: ubuntu-latest
     steps: