IDLE: add an AST browser

Feature or enhancement

As a companion to the proposed token browser (#152941), visualizing the abstract syntax tree helps in learning how Python parses code and in understanding the structure that the compiler works from.

I propose an AST Browser added as an extension on the Tools menu (Shell and editor). It shows ast.parse of the current window as a hierarchical, fully expanded ttk.Treeview, each node labeled with its type and inline simple fields.

Interaction:

• Selecting a node highlights the corresponding source in the editor.
• The browser follows the editor: the editor selection (or cursor) selects the single smallest enclosing node.
• Scope is the editor selection if it parses, else the Shell's current input, else the whole window (a context-dependent fragment that does not parse reports the syntax error).

It shares the window skeleton and editor-sync mechanism with the token browser, so the two are best reviewed and landed together.

A PR will follow.

Linked PRs

• gh-152947

Comienza revisando el issue complementario del explorador de tokens #152941 y el PR vinculado gh-152947, ya que este issue no menciona archivos de implementación ni pruebas. El trabajo estará completo cuando un AST Browser del menú Tools de IDLE analice el código fuente aplicable, muestre el árbol expandido, sincronice las selecciones con el editor e informe de errores de sintaxis para los fragmentos que no se puedan analizar.