Introduce a Roslyn source generator that emits the visitor boilerplate for
the C# AST from [DecompilerAstNode]-tagged node declarations: the
IAstVisitor interface, the AcceptVisitor overloads, the pattern-placeholder
nodes, and the initial DoMatch support. AccessorKind lets an accessor's
keyword be chosen independently of its role, an early step toward shedding
the NRefactory role model.