Revert "Removed circle CI (#4236)" (#4253)

This reverts commit e347b733c1.

.circleci/config.yml

@@ -0,0 +1,20 @@
+version: 2
+jobs:
+  build:
+    docker:
+      - image: circleci/node:12
+    steps:
+      - checkout
+      - restore_cache:
+          keys:
+            - dependencies-{{ checksum "yarn.lock" }}
+      - run:
+          name: Install
+          command: yarn install --pure-lockfile
+      - save_cache:
+          paths:
+            - node_modules
+          key: dependencies-{{ checksum "yarn.lock" }}
+      - run:
+          name: Check Prettier, ESLint, Flow
+          command: yarn ci-check