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

首先查看配套的 token browser issue #152941 和所链接的 PR gh-152947，因为此 issue 没有列出任何实现文件或测试。当 IDLE 的 Tools 菜单中的 AST Browser 能够解析适用的源代码、显示展开后的树、与编辑器同步选择，并报告无法解析片段的语法错误时，工作即完成。