Add xeus-cpp kernel (C++20) to conformance tests (#32)

Second of 7 xeus kernels requested in #30. Uses xcpp20 kernel (C++20 standard)
since xeus-cling already uses xcpp17.

Author: rgbkrk

.github/workflows/conformance.yml

@@ -73,6 +73,9 @@ jobs:
           - name: xeus-python
             install: micromamba install -y xeus-python -c conda-forge
             kernel-name: xpython
+          - name: xeus-cpp
+            install: micromamba install -y xeus-cpp -c conda-forge
+            kernel-name: xcpp20
 
     steps:
       - uses: actions/checkout@v4
@@ -112,7 +115,7 @@ jobs:
           go-version: '1.22'
 
       - name: Set up Mamba
-        if: contains(fromJson('["xeus-cling", "xeus-sql", "xeus-python"]'), matrix.kernel.name)
+        if: contains(fromJson('["xeus-cling", "xeus-sql", "xeus-python", "xeus-cpp"]'), matrix.kernel.name)
         uses: mamba-org/setup-micromamba@v2
         with:
           micromamba-version: 'latest'
@@ -151,7 +154,7 @@ jobs:
         run: ${{ matrix.kernel.install }}
 
       - name: Copy conda kernelspecs to user directory
-        if: contains(fromJson('["xeus-cling", "xeus-sql", "xeus-python"]'), matrix.kernel.name)
+        if: contains(fromJson('["xeus-cling", "xeus-sql", "xeus-python", "xeus-cpp"]'), matrix.kernel.name)
         run: |
           echo "Looking for kernelspecs..."
           find $MAMBA_ROOT_PREFIX -name "kernel.json" 2>/dev/null || true