chore: add lint script

Closes #3 (closed)

.gitlab-ci.yml

+image: node:14-alpine
+
 stages:
@@ -1,4 +3,4 @@
 stages:
-    - qa
+    - checks
 
 eslint:
@@ -3,6 +5,7 @@
 
 eslint:
+    stage: checks
     before_script:
         - yarn
     script:
         - yarn lint
@@ -5,4 +8,14 @@
     before_script:
         - yarn
     script:
         - yarn lint
+
+build:
+    stage: checks
+    before_script:
+        - yarn
+    script:
+        - cd packages/ra-cubicweb
+        - yarn build
+        - cd ../demo
+        - yarn build