Add GitLab CI config

.gitlab-ci.yml

+test:
+  stage: test
+  image: node:20
+  script:
+    - npm install
+    - npm run-script download_jar
+    - npm run-script package
+  artifacts:
+    expire_in: 1 week
+    paths:
+      - b-language-extension-*.vsix
+  cache:
+    paths:
+      - node_modules