Add CI job to test out-of-tree Pyodide builds

.github/workflows/emscripten.yml

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+# Attributed to NumPy https://github.com/numpy/numpy/pull/25894
+# https://github.com/numpy/numpy/blob/d2d2c25fa81b47810f5cbd85ea6485eb3a3ffec3/.github/workflows/emscripten.yml
+#
+
+name: Pyodide wheel
+
+on:
+  # TODO: refine after this is ready to merge
+  [push, pull_request, workflow_dispatch]
+
+env:
+  FORCE_COLOR: 3
+  PYODIDE_VERSION: 0.25.1
+  # PYTHON_VERSION and EMSCRIPTEN_VERSION are determined by PYODIDE_VERSION.
+  # The appropriate versions can be found in the Pyodide repodata.json
+  # "info" field, or in Makefile.envs:
+  # https://github.com/pyodide/pyodide/blob/main/Makefile.envs#L2
+  PYTHON_VERSION: 3.11.3
+  EMSCRIPTEN_VERSION: 3.1.46
+  NODE_VERSION: 18
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read  # to fetch code (actions/checkout)
+
+jobs:
+  build_wasm_emscripten:
+    name: Build and test Zarr for Pyodide
+    runs-on: ubuntu-22.04
+    # To enable this workflow on a fork, comment out:
+    # FIXME: uncomment after this is ready to merge
+    # if: github.repository == 'zarr-developers/zarr-python'
+    steps:
+      - name: Checkout Zarr repository
+        uses: actions/checkout@v4
+
+      - name: Set up Python ${{ env.PYTHON_VERSION }}
+        id: setup-python
+        uses: actions/setup-python@v5
+        with:
+          python-version: ${{ env.PYTHON_VERSION }}
+
+      - name: Set up Emscripten toolchain
+        uses: mymindstorm/setup-emsdk@v14
+        with:
+          version: ${{ env.EMSCRIPTEN_VERSION }}
+          actions-cache-folder: emsdk-cache
+
+      - name: Set up Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: ${{ env.NODE_VERSION }}
+
+      - name: Install pyodide-build
+        run: python -m pip install "pyodide-build==${{ env.PYODIDE_VERSION }}"
+
+      - name: Build Zarr for Pyodide
+        run: |
+          pyodide build
+
+      - name: Run Zarr tests for Pyodide
+        run: |
+          pyodide venv .venv-pyodide
+          source .venv-pyodide/bin/activate
+          python -m pip install dist/*.whl
+          python -m pip install pytest pytest-cov
+          python -m pytest -v --cov=zarr --cov-config=pyproject.toml zarr
+
+      - name: Upload Pyodide wheel artifact for debugging
+        # FIXME: Remove after this is ready to merge
+        uses: actions/upload-artifact@v4
+        with:
+          name: zarr-pyodide-wheel
+          path: dist/*.whl
+
+