chore: add vscode extensions volume to reduce ext reinstalls (ietf-tools#3833)

Author: NGPixel

.devcontainer/docker-compose.extend.yml

@@ -9,5 +9,9 @@ services:
             - /workspace/node_modules
             - /workspace/.parcel-cache
             - /workspace/__pycache__
+            - datatracker-vscode-ext:/root/.vscode-server/extensions
         # Runs app on the same network as the database container, allows "forwardPorts" in devcontainer.json function.
         network_mode: service:db
+
+volumes:
+    datatracker-vscode-ext: