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

Start with Python's tokenize output and IDLE's extension entry point on the Tools menu for the Shell and editor. Review how a ttk.Treeview can represent token type, string, and range, then verify the selection, double-click, cursor, and scope behavior described in the issue. Done means the browser follows the current window and maps token rows to source locations; gh-152946 is linked as the follow-up PR.