for #1240: Integrate XEP validation in CI

Adds a Github workflow that executes `tools/validate-xep0001-conformance.sh` against any changed XEP that was changed in a pull request.

.github/workflows/xep-validation.yml

@@ -0,0 +1,35 @@
+name: XEP validation
+
+on:
+  pull_request:
+    branches:
+      - master
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    name: Validate any XEP changes
+    steps:
+      - uses: actions/checkout@v3
+        with:
+          fetch-depth: 0
+
+      - name: Detect changes to XEP files
+        id: changed-xeps
+        uses: tj-actions/changed-files@v34
+        with:
+          files: |
+            xep-*.xml
+            inbox/*
+
+      - name: Validate changed file(s)
+        if: steps.changed-xeps.outputs.any_changed == 'true'
+        run: |
+          sudo apt-get install -y libxml2-utils
+          result=0
+          for xep in "${{ steps.changed-xeps.outputs.all_changed_files }}"; do
+            if ! tools/validate-xep0001-conformance.sh "$xep"; then
+              result=1
+            fi
+          done
+          exit $result