feature: Add Roslyn Analyzer for Fody plugin

[arontsang]

What kind of change does this PR introduce?

Feature. This adds a new Roslyn code analyzer that ensures that the [Reactive] attribute is added only to classes that implement IReactiveObject.

Unfortunately, I do not have the expertise to wrap this up in a nuget package

What is the current behavior?

When a user tries to add the [Reactive] attribute to a property on a class that does not implement IReactiveObject, the weaving silently fails.

What is the new behavior?

When a user tries to add the [Reactive] attribute to a property on a class that does not implement IReactiveObject, the IDE should give a compile time error.

What might this PR break?
None

Please check if the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

[todo]

Consider registering other actions that act on syntax instead of or in addition to symbols

ReactiveUI/src/ReactiveUI.Fody.Analyzer/ReactiveObjectAnalyzer.cs

Lines 55 to 60 in 61b7e15

	// TODO: Consider registering other actions that act on syntax instead of or in addition to symbols
	// See https://github.com/dotnet/roslyn/blob/master/docs/analyzers/Analyzer%20Actions%20Semantics.md for more information
	context.RegisterSyntaxNodeAction(AnalyzeAttribute, SyntaxKind.Attribute);
	}
	private static void AnalyzeAttribute(SyntaxNodeAnalysisContext context)

---

This comment was generated by todo based on a TODO comment in 61b7e15 in #2199. cc @arontsang.

[dnfclas]

All CLA requirements met.

[RLittlesII]

Currently the build script won't build or test this project. It should be added to the white list.

ReactiveUI/build.cake

Lines 16 to 30 in 1357b14

	var packageWhitelist = new List<FilePath>
	{
	MakeAbsolute(File("./src/ReactiveUI/ReactiveUI.csproj")),
	MakeAbsolute(File("./src/ReactiveUI.Testing/ReactiveUI.Testing.csproj")),
	MakeAbsolute(File("./src/ReactiveUI.Events/ReactiveUI.Events.csproj")),
	MakeAbsolute(File("./src/ReactiveUI.Events.XamEssentials/ReactiveUI.Events.XamEssentials.csproj")),
	MakeAbsolute(File("./src/ReactiveUI.Events.XamForms/ReactiveUI.Events.XamForms.csproj")),
	MakeAbsolute(File("./src/ReactiveUI.Fody/ReactiveUI.Fody.csproj")),
	MakeAbsolute(File("./src/ReactiveUI.Fody.Helpers/ReactiveUI.Fody.Helpers.csproj")),
	MakeAbsolute(File("./src/ReactiveUI.AndroidSupport/ReactiveUI.AndroidSupport.csproj")),
	MakeAbsolute(File("./src/ReactiveUI.AndroidX/ReactiveUI.AndroidX.csproj")),
	MakeAbsolute(File("./src/ReactiveUI.XamForms/ReactiveUI.XamForms.csproj")),
	MakeAbsolute(File("./src/ReactiveUI.Uno/ReactiveUI.Uno.csproj")),
	MakeAbsolute(File("./src/ReactiveUI.Blazor/ReactiveUI.Blazor.csproj")),
	};