add continuous integration (linter)

.gitlab-ci.yml

+workflow:
+  rules:
+    - if: $CI_PIPELINE_SOURCE == 'merge_request_event' || $CI_PIPELINE_SOURCE == 'schedule' || $CI_PIPELINE_SOURCE == 'web'
+    # See https://docs.gitlab.com/ee/ci/yaml/workflow.html#switch-between-branch-pipelines-and-merge-request-pipelines
+    - if: $CI_COMMIT_BRANCH && $CI_OPEN_MERGE_REQUESTS == null && $CI_COMMIT_BRANCH !~ /^docs\//
+variables:
+  GIT_DEPTH: '5'
+  DEFAULT_NODE_VERSION: '16'
+  LINUX_DISTRO: bullseye
+  NPM_CONFIG_CACHE: &npm_cache_dir .cache/npm
+  NPM_CONFIG_FUND: 'false'
+  NPM_CONFIG_PREFER_OFFLINE: 'true'
+default:
+  image: node:$DEFAULT_NODE_VERSION-$LINUX_DISTRO
+  interruptible: true
+.rules:
+  - &if_docs_mr
+    rules:
+      - if: &docs_mr $CI_PIPELINE_SOURCE == 'merge_request_event' && $CI_MERGE_REQUEST_SOURCE_BRANCH_NAME =~ /^docs\//
+  - &unless_docs_mr
+    rules:
+      - if: *docs_mr
+        when: never
+      - if: $CI_JOB_NAME == 'lint' && $CI_PIPELINE_SOURCE == 'push' && $CI_PROJECT_PATH != 'antora/antora-lunr-extension'
+        when: manual
+      - when: on_success
+lint:
+  stage: .pre
+  <<: *unless_docs_mr
+  before_script:
+    - node -p '`${os.type()} ${os.release()}\nNode.js ${process.version}`'
+    - npm ci --quiet
+  cache: &npm_cache
+    key: npm-cache
+    paths:
+      - *npm_cache_dir
+    policy: pull
+  script:
+    - npm run lint
+# this job signals success to the MR UI
+docs:
+  stage: test
+  <<: *if_docs_mr
+  script: echo 'we love docs!'