Refine project structure

[shibayan]

This pull request introduces several updates focused on dependency upgrades, solution/project structure, and significant improvements to test coverage for file preview handlers. The most notable changes include updating NuGet package versions, restructuring the solution file, and rewriting and expanding tests for various file preview handlers.

Dependency and Package Updates:

• Upgraded Microsoft.Extensions.DependencyInjection from version 10.0.3 to 10.0.5 in WinQuickLook.App.csproj and updated related entries in packages.lock.json to ensure consistency. [1] [2] [3]
• Updated Microsoft.Web.WebView2 from version 1.0.3800.47 to 1.0.3856.49 in WinQuickLook.Core.csproj and corresponding lock files. [1] [2] [3] [4]

Solution and Project Structure:

• Reorganized the solution file: replaced the old flat WinQuickLook.slnx structure with a new one that groups projects under /src/ and /tests/ folders for better clarity and maintainability. [1] [2]

Test Suite Improvements:

• Rewrote and expanded tests for CodeFilePreviewHandler, HtmlFilePreviewHandler, and introduced new tests for FilePreviewHandler, GenericFilePreviewHandler, and SvgFilePreviewHandler. These new tests provide more thorough coverage, including positive/negative cases and behavioral assertions. [1] [2] [3] [4] [5]
• Removed obsolete or replaced test files for CodeFilePreviewHandler and HtmlFilePreviewHandler to avoid redundancy. [1] [2]
• Improved test naming for clarity, e.g., renamed the FitTo test to FitTo_SizeExceedsLimit_ReturnsScaledSize for better intent description.