Switches testing library to Jest, adds test workflow

.github/workflows/test.yml

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+name: CI
+on: push
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/setup-node@v2
+        with:
+          node-version: "15"
+          check-latest: true
+      - uses: actions/checkout@v2
+      - name: Install modules
+        run: npm i
+      - name: Run tests
+        run: npm run test

.gitignore

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+.DS_Store
+node_modules
+package-lock.json

.prettierrc

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+{
+  "printWidth": 120
+}