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

Start by reviewing the companion token browser issue #152941 and the linked PR gh-152947, since this issue names no implementation files or tests. The work is complete when an IDLE Tools-menu AST Browser parses the applicable source, displays the expanded tree, synchronizes selections with the editor, and reports syntax errors for unparseable fragments.