Add `Licensed` To Help Verify Prod Licenses (#180)

* add licensed workflow and config files

* manual dependency verification

* Hide generated license files in pr

.gitattributes

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+.licenses/** -diff linguist-generated=true

.github/workflows/licensed.yml

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+name: Licensed
+
+on:
+  push: {branches: main}
+  pull_request: {branches: main}
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    name: Check licenses
+    steps:
+      - uses: actions/checkout@v2
+      - run: npm ci
+      - name: Install licensed
+        run: |
+          cd $RUNNER_TEMP
+          curl -Lfs -o licensed.tar.gz https://github.com/github/licensed/releases/download/2.12.2/licensed-2.12.2-linux-x64.tar.gz
+          sudo tar -xzf licensed.tar.gz
+          sudo mv licensed /usr/local/bin/licensed
+      - run: licensed status

.licensed.yml

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+sources:
+  npm: true
+
+allowed:
+  - apache-2.0
+  - bsd-2-clause
+  - bsd-3-clause
+  - isc
+  - mit
+  - cc0-1.0
+  - unlicense
+
+reviewed:
+  npm: