Create Danger SwiftLint/SwiftFormat GHA workflow

.github/workflows/danger.yml

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+# This is a basic workflow to help you get started with Actions
+
+name: Danger
+
+# Controls when the action will run. Triggers the workflow on push or pull request
+# events but only for the master branch
+on:
+  pull_request:
+    branches: [ master ]
+
+# A workflow run is made up of one or more jobs that can run sequentially or in parallel
+jobs:
+  danger-lint:
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+
+    - name: Danger Swift
+      uses: maxdesiatov/danger-swift@swiftlint-docker
+      env:
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}