.github: add shellcheck workflow (#51)

Make CI lint shell scripts with shellcheck [1], following up from the
previous commit [2].

[1] https://github.com/koalaman/shellcheck
[2] f2b7edd, 2023-08-24, "bin(run-tests, run): resolve shellcheck complaints"

Author: ee7

.github/workflows/shellcheck.yml

@@ -0,0 +1,14 @@
+name: shellcheck
+
+on: [push, pull_request, workflow_dispatch]
+
+jobs:
+  shellcheck:
+    name: Run shellcheck on scripts
+    runs-on: ubuntu-22.04
+    steps:
+      - name: Checkout
+        uses: actions/checkout@c85c95e3d7251135ab7dc9ce3241c5835cc595a9
+
+      - name: Run shellcheck
+        uses: ludeeus/action-shellcheck@00cae500b08a931fb5698e11e79bfbd38e612a38