Add multi-version byte-compile job and treat warnings as errors

Add byte-compile matrix (27.2, 29.4, snapshot) back to ci.yml since
melpazoid runs in Docker with a fixed Emacs version. Set
WARN_IS_ERROR=true in the melpazoid workflow.

Author: dakra

.github/workflows/ci.yml

@@ -6,6 +6,27 @@ on:
   pull_request:
 
 jobs:
+  byte-compile:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        emacs_version:
+          - '27.2'
+          - '29.4'
+          - 'snapshot'
+    steps:
+      - uses: actions/checkout@v4
+      - uses: purcell/setup-emacs@master
+        with:
+          version: ${{ matrix.emacs_version }}
+
+      - name: Byte-compile
+        run: |
+          emacs --batch -Q -L . \
+            -f batch-byte-compile \
+            ghostel.el ghostel-debug.el
+
   test:
     runs-on: ubuntu-latest
     strategy:

.github/workflows/melpazoid.yml

@@ -21,4 +21,5 @@ jobs:
           LOCAL_REPO: ${{ github.workspace }}
           RECIPE: (ghostel :fetcher github :repo "dakra/ghostel")
           EXIST_OK: false
+          WARN_IS_ERROR: true
         run: make -C ~/melpazoid