Add .slnx file for .NET 9+ compatibility alongside existing .sln file

[Copilot]

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

Original prompt

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)

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