Add xUnit.net v3 compatibility

[bdovaz]

Fixes #45

• Change the “taskName” property (deprecated) to “label” in .vscode/tasks.json
• Do not version .DS_Store
• Use a source generator to generate V2 attributes and discoverers
• Use a source generator to generate V3 attributes
• Use a source generator to generate V2 tests
• Use a source generator to generate V3 tests
• Use a file called Xunit.Categories.txt with AdditionalFile for use by source generators. This allows users to freely add new attributes without having to rely on PRs in the repository and the release of a new package if the source generators are distributed through NuGet in the future.
• Create tests for source generators
• Target only netstandard2.0 based on the recommendations at: https://github.com/dotnet/designs/blob/main/accepted/2024/net-standard-recommendation.md
• Remove nuspec files: https://learn.microsoft.com/en-us/nuget/reference/nuspec

A .nuspec file is not required to create packages for SDK-style projects (typically .NET Core and .NET Standard projects that use the SDK attribute). (Note that a .nuspec is generated when you create the package.)

• I changed the version to 4.0.0 (v2) and changed the release notes
• I changed the version to 1.0.0 (v3) and changed the release notes
• I created a Directory.Builds.props to share common attributes
• I added the license with PackageLicenseExpression
• I updated the README.md
• I have updated the workflows, but I am unsure if I have done so correctly as I am unable to test it.
• I have updated NuGet package dependencies.

[bdovaz]

@brendanconnolly the error comes from: https://github.com/marocchino/sticky-pull-request-comment/issues?q=%22Resource%20not%20accessible%20by%20integration%22

I guess that's something you'll have to sort out yourself, as I don't have permission to carry out the kind of operations mentioned in the issues.

[bdovaz]

@brendanconnolly can you review, please? Thanks

If for whatever reason you are not interested in continuing with this project, I offer my services as a collaborator. I prefer that to having to create a fork and complicating everything further.