Upgrade to NET8

[bwinsley]

Original PR was merged into master without squashing. @oliverbock and I attempted a squash and force push which somehow broke the git history. Master has since been restored so I am redoing this PR which we will squash and rebase.

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This pull request modernizes the project by upgrading it from .NET Framework 4.7.2 to .NET 8, updating build configurations, and improving memory management for managed JavaScript function wrappers. It also revises documentation and sample code to reflect these changes, ensuring compatibility with current development tools and practices.

.NET 8 Migration and Build System Updates:

• Migrated Fiddling.csproj and JavaScript.Net.vcxproj to .NET 8 SDK-style projects, removed legacy .NET Framework settings, and updated platform/toolset requirements for C++/CLI to use CLRSupport=NetCore. [1] [2]
• Updated solution file JavaScript.Net.sln for Visual Studio 2022/2019 and removed references to older Visual Studio versions.
• Removed obsolete configuration file app.config and legacy assembly references, as .NET 8 includes most system assemblies by default. [1] [2]

Changes to method of tracking and garbage collecting JavascriptExternals

• Passing GCHandle to unmanaged c++ code would cause issues in NET8 and accessing methods on it such as GCHandle.Target would throw System.AccessViolationException.
• The solution was to use the method GCHandle.ToIntPtr to obtain a pointer which we can pass to unmanaged code

Memory Management Enhancements:

• Added a dedicated wrapper (JavascriptFunctionWrapper and WrappedJavascriptFunction) for managed JavaScript function objects, enabling proper cleanup of V8 handles and managed references when the context is disposed. Updated construction and disposal logic in both header and implementation files. [1] [2] [3] [4]
• Readded memory management code into Program.cs in Fidding.csproj [1] [2] [3]
• This change mimics that of Reduce memory inefficiencies and leaks #95
• See performance improvement below

Before:
After:

This upgrade brings the project up to date with current .NET and Visual Studio standards, improves reliability and maintainability, and provides clearer guidance for users and contributors.