ci: add shellcheck job

.github/workflows/shellcheck.yml

@@ -0,0 +1,19 @@
+name: Shellcheck
+on:
+  pull_request:
+    branches:
+      - master
+jobs:
+  shellcheck:
+    name: Shellcheck
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - name: Run ShellCheck
+      uses: ludeeus/action-shellcheck@2.0.0
+      env:
+        SHELLCHECK_OPTS: -x # allow outside sources
+      with:
+        # This code comes directly from upstream libsecp256k1
+        # and should not be linted here.
+        ignore_paths: ./secp256k1-sys/depend/**/*.sh