add ci script

.gitlab.ci.yml

+build-firefox-extension:
+  image: node:lts
+  before_script:
+    - yarn global add web-ext
+  script:
+    - yarn install
+    - yarn build
+    - yarn web-ext sign --api-key=${MOZ_API_KEY} --api-secret=${MOZ_API_SECRET} --id=${MOZ_GECKO_ID} -s src/
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