Add `Node` union

Add a new Node union that is an enum over all node types. This can be useful when implementing methods that can operate on any node. For example, Ruff's formatter has (roughly) the API format(node: AnyNode) -> String

I haven't figured out the full requirements yet, and I don't know yet if we'll need both the owned and reference versions:

#[derive(Debug, Clone, PartialEq, Eq, Hash)]
enum Node {
	IfStmt(ast::IfStmt),
	ConstantExpr(ast::ConstantExpr),
	...
}

#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash, is_macro)]
enum NodeRef<'a> {
	IfStmt(&'a ast::IfStmt),
	ConstantExpr(&'a ast::ConstantExpr)
	...
}

Note: An alternative naming more in line with Path and PathBuf would be to name the types NodeBuf (owned) and Node (reference)

The enums should have the following basic methods:

• if_stmt(self) -> Option<ast::IfStmt>
• as_if_stmt(&self) -> Option<&ast::IfStmt>
• const is_if_stmt(&self) -> bool

Node could also implement AsRef that returns a NodeRef

I may have time to work on this sometime soon but I wanted to put this up for discussion first to get feedback.

Revise os tipos de nós AST existentes e compare os designs propostos de Node proprietário e NodeRef de referência. Resolva os requisitos de nomenclatura e propriedade antes de implementar o enum, os métodos de conversão e as verificações de tipo. O trabalho estará concluído quando os tipos de nós compatíveis e a API necessária estiverem acordados e cobertos por testes.