Add concurrency setup

Author: backslash-f

.github/workflows/swift.yml

@@ -1,12 +1,16 @@
 ---
-name: "ci"
+name: "CI"
 
 on:
   push:
     branches: [main]
   pull_request:
     branches: [main]
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
 jobs:
   test:
     runs-on: macOS-latest