chore: Add matrix notify workflow

.github/workflows/matrix_notify.yaml

@@ -0,0 +1,49 @@
+name: Matrix Notification
+
+on:
+  issues:
+    types: [opened]
+  pull_request:
+    types: [opened]
+
+jobs:
+  notify:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Send notification to Matrix room
+        env:
+          HOMESERVER: ${{ secrets.MATRIX_HOMESERVER }}
+          ACCESS_TOKEN: ${{ secrets.MATRIX_ACCESS_TOKEN }}
+          ROOM_ID: ${{ secrets.MATRIX_ROOM_ID }}
+          EVENT_NAME: ${{ github.event_name }}
+          ISSUE_TITLE: ${{ github.event.issue.title }}
+          ISSUE_URL: ${{ github.event.issue.html_url }}
+          PR_TITLE: ${{ github.event.pull_request.title }}
+          PR_URL: ${{ github.event.pull_request.html_url }}
+        run: |
+          if [ "${EVENT_NAME}" = "issues" ]; then
+            PREFIX="New issue"
+            TITLE="${ISSUE_TITLE}"
+            URL="${ISSUE_URL}"
+          else
+            PREFIX="New pull request"
+            TITLE="${PR_TITLE}"
+            URL="${PR_URL}"
+          fi
+
+          PLAIN="${PREFIX}: ${TITLE} - ${URL}"
+          HTML="${PREFIX}: <b>${TITLE}</b> - <a href=\"${URL}\">${URL}</a>"
+          TXN_ID=$(date +%s%N)
+
+          curl -s -o /dev/null -w "%{http_code}" -X PUT \
+            "${HOMESERVER}/_matrix/client/v3/rooms/${ROOM_ID}/send/m.room.message/${TXN_ID}" \
+            -H "Authorization: Bearer ${ACCESS_TOKEN}" \
+            -H "Content-Type: application/json" \
+            -d @- <<EOF
+          {
+            "msgtype": "m.text",
+            "body": "${PLAIN}",
+            "format": "org.matrix.custom.html",
+            "formatted_body": "${HTML}"
+          }
+          EOF