Preparations for the 1.0 release

* Less cluttered solution layout.
* Move examples to a repository of its own.
* Move Roslyn analyzer to a repository of its own.
* Enable central package management.
* Clean up csproj files.
* Add README file to NuGet packages.

src/Extensions/Spectre.Console.Json/Syntax/JsonSyntax.cs

@@ -0,0 +1,9 @@
+namespace Spectre.Console.Json.Syntax;
+
+/// <summary>
+/// Represents a syntax node in the JSON abstract syntax tree.
+/// </summary>
+public abstract class JsonSyntax
+{
+    internal abstract void Accept<T>(JsonSyntaxVisitor<T> visitor, T context);
+}