IDLE: add a token browser

Seeing how source is split into tokens helps in learning how Python's tokenizer works and in debugging tokenizer-dependent behavior, but IDLE offers no way to inspect the token stream.

I propose a Token Browser added as an extension on the Tools menu (Shell and editor). It shows the output of tokenize for the current window in a ttk.Treeview — one row per token with its type, string, and range.

Interaction:

• Selecting a row highlights the corresponding source in the editor.
• Double-clicking a row moves the editor cursor to that token.
• The browser follows the editor: a selection selects every overlapping token row, and the cursor alone selects the single token containing it.
• Scope is the editor selection if there is one, else the Shell's current input, else the whole window.

A PR will follow.

Linked PRs

• gh-152946

从 Python 的 tokenize 输出以及 Shell 和编辑器的 Tools 菜单中的 IDLE 扩展入口点开始。检查 ttk.Treeview 如何表示令牌类型、字符串和范围，然后验证 issue 中描述的选择、双击、光标和作用域行为。完成的标准是浏览器跟随当前窗口，并将令牌行映射到源代码位置；gh-152946 已作为后续 PR 链接。