Add .slnx file for .NET 9+ compatibility alongside existing .sln file (#162)

This PR adds a modern XML-based solution file (`.slnx`) to the
repository using the `dotnet solution migrate` command, as requested in
the issue. The `.slnx` format is the new solution file format introduced
in .NET 9 SDK.

## Changes

- Generated `IntelliTect.Multitool.slnx` using `dotnet solution migrate
IntelliTect.Multitool.sln`
- The existing `IntelliTect.Multitool.sln` file is retained for
backwards compatibility with older versions of Visual Studio and .NET
SDK

## Benefits

The `.slnx` format provides several advantages over the traditional
`.sln` format:
- **Human-readable XML structure** - easier to understand and parse
- **Better for version control** - cleaner diffs and merge conflicts
- **Modern format** - aligned with .NET 9+ tooling

## Compatibility

Both solution files now coexist in the repository:
- **`.sln` file** - maintains compatibility with .NET 8 SDK, Visual
Studio 2022, and earlier versions
- **`.slnx` file** - provides modern solution format for .NET 9+ SDK and
future Visual Studio versions

The repository continues to build successfully with .NET 8 SDK, ensuring
no breaking changes for existing workflows.

## References

- [Microsoft Documentation: dotnet sln
migrate](https://learn.microsoft.com/en-us/dotnet/core/tools/dotnet-sln#migrate)

<!-- START COPILOT CODING AGENT SUFFIX -->



<details>

<summary>Original prompt</summary>

> run `dotnet sln migrate`
(https://learn.microsoft.com/en-us/dotnet/core/tools/dotnet-sln#migrate)
to add in a .slnx file to live alongside our .sln file for now (.sln for
backwards compatibility for now)


</details>


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Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: BenjaminMichaelis <[email protected]>

Author: Copilot

IntelliTect.Multitool.slnx

@@ -0,0 +1,12 @@
+<Solution>
+  <Folder Name="/Solution Items/">
+    <File Path=".github/workflows/build-and-test.yml" />
+    <File Path=".github/workflows/deploy.yml" />
+    <File Path="Directory.Build.props" />
+    <File Path="Directory.Packages.props" />
+    <File Path="global.json" />
+    <File Path="README.md" />
+  </Folder>
+  <Project Path="IntelliTect.Multitool.Tests/IntelliTect.Multitool.Tests.csproj" />
+  <Project Path="IntelliTect.Multitool/IntelliTect.Multitool.csproj" />
+</Solution>