Add web-ext-lint job

Change-Id: I12018ea457599ba0dd611c05464f3c1dd14d0fba
diff --git a/playbooks/web-ext-lint/pre.yaml b/playbooks/web-ext-lint/pre.yaml
new file mode 100644
index 0000000..ccd2fad
--- /dev/null
+++ b/playbooks/web-ext-lint/pre.yaml
@@ -0,0 +1,4 @@
+- name: Ensure web-ext is installed
+  hosts: all
+  roles:
+    - ensure-web-ext