ci: Replace js78 job

Provided we use the correct version of the SpiderMonkey shell, we
can perform checks using the same engine that is used by gjs.

However some engine features are opt-in, so the set of features enabled
by gjs and js91 may differ. The obvious option for avoiding this is
replacing js91 with gjs for tests.

Switch to the newly added gjs-check-syntax script, which does precisely
that.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/215>

.gitlab-ci.yml

@@ -26,7 +26,6 @@ default:
 variables:
   FDO_UPSTREAM_REPO: GNOME/gnome-shell-extensions
   LINT_LOG: "eslint-report.xml"
-  JS_LOG: "js-report.txt"
 
 workflow:
   rules:
@@ -103,12 +102,7 @@ js_check:
   stage: review
   <<: *prereview_req
   script:
-    - find extensions -name '*.js' -exec js78 -c '{}' ';' 2>&1 | tee $JS_LOG
-    - (! grep -q . $JS_LOG)
-  artifacts:
-    paths:
-      - ${JS_LOG}
-    when: on_failure
+    - gjs-check-syntax
 
 eslint:
   stage: review