Switch to .slnx format for solution configuration

[clairernovotny]

No description provided.

[Copilot]

Pull Request Overview

This PR modernizes the solution configuration by switching from the traditional Visual Studio solution file format (.sln) to the newer XML-based .slnx format, while removing the associated ReSharper configuration file.

• Migrated solution configuration from legacy .sln format to modern .slnx XML format
• Removed ReSharper-specific configuration file (.sln.DotSettings)
• Preserved all project references and solution folder structure

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/Humanizer.slnx	New XML-based solution file with projects and solution items configuration
src/Humanizer.sln.DotSettings	Removed ReSharper configuration settings file
src/Humanizer.sln	Removed legacy Visual Studio solution file

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