Add CodeQL workflow and fix C# language version

Co-authored-by: trackd <17672644+trackd@users.noreply.github.com>

Author: Copilot

.github/workflows/codeql.yml

@@ -0,0 +1,56 @@
+name: "CodeQL"
+
+on:
+  push:
+    branches: [ "main" ]
+  pull_request:
+    branches: [ "main" ]
+  schedule:
+    - cron: '15 5 * * 2'
+
+jobs:
+  analyze:
+    name: Analyze
+    runs-on: ${{ (matrix.language == 'swift' && 'macos-latest') || 'ubuntu-latest' }}
+    timeout-minutes: ${{ (matrix.language == 'swift' && 120) || 360 }}
+    permissions:
+      # required for all workflows
+      security-events: write
+      # only required for workflows in private repositories
+      actions: read
+      contents: read
+
+    strategy:
+      fail-fast: false
+      matrix:
+        language: [ 'csharp', 'powershell' ]
+
+    steps:
+    - name: Checkout repository
+      uses: actions/checkout@v4
+
+    - name: Initialize CodeQL
+      uses: github/codeql-action/init@v3
+      with:
+        languages: ${{ matrix.language }}
+        queries: security-extended,security-and-quality
+
+    - name: Setup .NET
+      if: matrix.language == 'csharp'
+      uses: actions/setup-dotnet@v4
+      with:
+        dotnet-version: '8.0.x'
+
+    - name: Setup PowerShell
+      if: matrix.language == 'powershell'
+      shell: pwsh
+      run: |
+        Write-Host "PowerShell version: $($PSVersionTable.PSVersion)"
+
+    - name: Autobuild
+      uses: github/codeql-action/autobuild@v3
+
+    - name: Perform CodeQL Analysis
+      uses: github/codeql-action/analyze@v3
+      with:
+        category: "/language:${{matrix.language}}"

src/Sixel/Sixel.csproj

@@ -5,7 +5,7 @@
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
-    <LangVersion>13.0</LangVersion>
+    <LangVersion>12.0</LangVersion>
     <Version>0.5.0</Version>
     <AnalysisMode>Recommended</AnalysisMode>
     <EnableNETAnalyzers>true</EnableNETAnalyzers>