Add actionlint

- Add GitHub Actions workflow to lint workflows with actionlint.
- Configure Visual Studio Code extensions.

.github/actionlint-matcher.json

@@ -0,0 +1,17 @@
+{
+  "problemMatcher": [
+    {
+      "owner": "actionlint",
+      "pattern": [
+        {
+          "regexp": "^(?:\\x1b\\[\\d+m)?(.+?)(?:\\x1b\\[\\d+m)*:(?:\\x1b\\[\\d+m)*(\\d+)(?:\\x1b\\[\\d+m)*:(?:\\x1b\\[\\d+m)*(\\d+)(?:\\x1b\\[\\d+m)*: (?:\\x1b\\[\\d+m)*(.+?)(?:\\x1b\\[\\d+m)* \\[(.+?)\\]$",
+          "file": 1,
+          "line": 2,
+          "column": 3,
+          "message": 4,
+          "code": 5
+        }
+      ]
+    }
+  ]
+}

.github/workflows/lint.yml

@@ -0,0 +1,39 @@
+name: lint
+
+on:
+  push:
+    branches: [ main ]
+    paths-ignore:
+      - '**/*.gitattributes'
+      - '**/*.gitignore'
+      - '**/*.md'
+  pull_request:
+    branches:
+      - main
+      - dotnet-vnext
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+env:
+  FORCE_COLOR: 3
+  TERM: xterm
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+
+    steps:
+
+    - name: Checkout code
+      uses: actions/checkout@3df4ab11eba7bda6032a0b82a6bb43b11571feac # v4.0.0
+
+    - name: Lint workflows
+      shell: bash
+      env:
+        ACTIONLINT_VERSION: '7b75d16d41920ec126e6f3269db0c6f3ab613c38' # v1.6.25
+      run: |
+        echo "::add-matcher::.github/actionlint-matcher.json"
+        bash <(curl --silent --show-error "https://raw.githubusercontent.com/rhysd/actionlint/${ACTIONLINT_VERSION}/scripts/download-actionlint.bash")
+        ./actionlint -color

.vscode/extensions.json

@@ -0,0 +1,7 @@
+{
+  "recommendations": [
+    "editorconfig.editorconfig",
+    "ms-dotnettools.csharp",
+    "ms-vscode.PowerShell"
+  ]
+}