Replace Danger with SwiftLint to improve warnings (#293)

Looks like the Danger action duplicates warning comments. The new SwiftLint action does not, although warnings only show up in the diff view, which I think is an acceptable trade-off.

.github/workflows/danger.yml

@@ -1,23 +0,0 @@
-# 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 main branch
-on:
-  pull_request:
-    branches: [main]
-
-# A workflow run is made up of one or more jobs that can run sequentially or in parallel
-jobs:
-  danger-lint:
-    runs-on: ubuntu-20.04
-
-    steps:
-      - uses: actions/checkout@v2
-
-      - name: Danger Swift
-        uses: maxdesiatov/danger-swift@swiftlint-docker
-        if: github.event.pull_request.head.repo.full_name == github.repository
-        env:
-          GITHUB_TOKEN: ${{ secrets.PAT }}

.github/workflows/swiftlint.yml

@@ -0,0 +1,18 @@
+name: SwiftLint
+
+on:
+  pull_request:
+    paths:
+      - ".github/workflows/swiftlint.yml"
+      - ".swiftlint.yml"
+      - "**/*.swift"
+
+jobs:
+  SwiftLint:
+    runs-on: ubuntu-20.04
+    steps:
+      - uses: actions/checkout@v1
+      - name: GitHub Action for SwiftLint
+        uses: norio-nomura/action-swiftlint@3.1.0
+        env:
+          DIFF_BASE: ${{ github.base_ref }}