ci: Add "source_check" stage

More testing is always good, and the static analysis that eslint
provides goes well beyond what js60 offers, so run it as part of
the CI.

This will also ensure that new contributions comply with the style
rules we have set up.

https://gitlab.gnome.org/GNOME/gnome-shell-extensions/merge_requests/50

.gitlab-ci.yml

@@ -1,6 +1,20 @@
 stages:
+ - source_check
  - build
 
+variables:
+ LINT_LOG: "eslint-report.txt"
+
+eslint:
+  image: registry.gitlab.gnome.org/gnome/gjs:fedora.static-analysis
+  stage: source_check
+  script:
+    - eslint -o $LINT_LOG -c lint/eslintrc-legacy.json --no-color . || { cat $LINT_LOG; false; }
+  artifacts:
+    paths:
+      - ${LINT_LOG}
+    when: on_failure
+
 build-shell-extensions:
   image: fedora:latest
   stage: build