Add publiccode.yml check as a GitHub workflow

Check the publiccode.yml is still valid and adheres to the spec whenever
it's changed.

.github/workflows/publiccode-yml-validation.yml

@@ -0,0 +1,18 @@
+name: publiccode.yml validation
+
+on:
+    pull_request:
+        paths:
+            - publiccode.yml
+    push:
+        paths:
+            - publiccode.yml
+
+jobs:
+    publiccode_yml_validation:
+        runs-on: ubuntu-latest
+        steps:
+            - uses: actions/checkout@v4
+            - uses: italia/publiccode-parser-action@v1
+              with:
+                  publiccode: "publiccode.yml"