Migrate to .editorconfig from ruleset files (OSOE-179)

[Piedone]

This is only suitable once .editorconfig has everything ruleset files provide already:

• IDE support, i.e. you don't have to manually add every rule but can just select what to do for each rule. This is now done with VS 2022. However, it only works for files under the solution, so you can't edit under the tools folder.
• Ability to extend another .editorconfig file. This is only interesting for overriding our default rules though (as we explain here). It would also help to get rid of the editorconfig copy to the solution root. However, perhaps the <EditorConfigFiles> could help us, see this blog post. An alternative would be https://github.com/dotnet/roslyn/issues/19028#issuecomment-843571161.
• There's also .globalconfig, but that's only for analyzer configs, not edit configuration like charset or guidelines. See https://github.com/dotnet/roslyn/issues/47707.

Tasks:

• [x] Migrate the config files, see the official docs.
• [x] Perhaps we need to have use case-specific packages, e.g. `Lombiq.Analyzers.OrchardCore"?
• [x] Update docs. Search for ".editorconfig" and "ruleset".
• [x] Add docs on migration, including converting ruleset files.
• [x] Check if we need <developmentDependency>true</developmentDependency> in the nuspec template. The analyzer packages we use do this. Check the generated <PackageReference> after a new install and change the docs, including release notes, and consuming projects if necessary.
• [x] Publish new NuGet package. This will need to be a major release. Add migration docs to the release.

[Piedone]

Some notes:

• <GlobalAnalyzerConfigFiles> and <EditorConfigFiles> seem to be virtually equivalent. Even if you use the latter and name your files something.editorconfig, they'll actually be global AnalyzerConfig files and need is_global = true. GlobalAnalyzerConfigFiles is meant to be used by us.
• While .editorconfig files work solely based on the folder structure, .globalconfigs can do this and also be referenced with <GlobalAnalyzerConfigFiles>.
• We could leave the editor-specific config, like charset, guidelines, tabs, in .editorconfig, which would remain copied. Analyzer configs would be moved to .globalconfigs, and would be referenced instead of copied.

[Piedone]

We'll have a hard time getting the editor GUI working: https://github.com/dotnet/roslyn/issues/70326

[Piedone]

We need .NET 8 for this: https://github.com/dotnet/roslyn-analyzers/issues/6281#issuecomment-1381351146

[Piedone]

We don't need .NET 8, that's only when TreatWarningsAsErrors=false and CodeAnalysisTreatWarningsAsErrors=true but we don't do that.