Migrate solution to SLNX

Migrates the .NET solution to use the new [`.slnx`][slnx] file format.

[slnx]: https://devblogs.microsoft.com/dotnet/introducing-slnx-support-dotnet-cli/

Contributes to martincostello/repo-migrations#85.

Signed-off-by: costellobot[bot] <102247573+costellobot[bot]@users.noreply.github.com>

Author: costellobot[bot]

TodoApp.sln

@@ -0,0 +1,53 @@
+<Solution>
+  <Folder Name="/perf/">
+    <Project Path="perf/TodoApp.Benchmarks/TodoApp.Benchmarks.csproj" />
+  </Folder>
+  <Folder Name="/Solution Items/">
+    <File Path=".editorconfig" />
+    <File Path=".gitattributes" />
+    <File Path=".gitignore" />
+    <File Path=".markdownlint.json" />
+    <File Path=".vsconfig" />
+    <File Path="build.ps1" />
+    <File Path="CODE_OF_CONDUCT.md" />
+    <File Path="Directory.Build.props" />
+    <File Path="global.json" />
+    <File Path="LICENSE" />
+    <File Path="NuGet.config" />
+    <File Path="omnisharp.json" />
+    <File Path="README.md" />
+    <File Path="SECURITY.md" />
+    <File Path="startvs.cmd" />
+    <File Path="startvscode.cmd" />
+    <File Path="TodoApp.ruleset" />
+  </Folder>
+  <Folder Name="/Solution Items/.github/">
+    <File Path=".github/actionlint-matcher.json" />
+    <File Path=".github/CODEOWNERS" />
+    <File Path=".github/CONTRIBUTING.md" />
+    <File Path=".github/dependabot.yml" />
+    <File Path=".github/FUNDING.yml" />
+    <File Path=".github/ISSUE_TEMPLATE.md" />
+    <File Path=".github/PULL_REQUEST_TEMPLATE.md" />
+  </Folder>
+  <Folder Name="/Solution Items/.github/ISSUE_TEMPLATE/">
+    <File Path=".github/ISSUE_TEMPLATE/bug_report.md" />
+    <File Path=".github/ISSUE_TEMPLATE/config.yml" />
+  </Folder>
+  <Folder Name="/Solution Items/.github/workflows/">
+    <File Path=".github/workflows/build.yml" />
+    <File Path=".github/workflows/code-scan.yml" />
+    <File Path=".github/workflows/lint.yml" />
+  </Folder>
+  <Folder Name="/Solution Items/.vscode/">
+    <File Path=".vscode/extensions.json" />
+    <File Path=".vscode/launch.json" />
+    <File Path=".vscode/tasks.json" />
+  </Folder>
+  <Folder Name="/src/">
+    <Project Path="src/TodoApp/TodoApp.csproj" />
+  </Folder>
+  <Folder Name="/tests/">
+    <Project Path="tests/TodoApp.Tests/TodoApp.Tests.csproj" />
+  </Folder>
+</Solution>