IDLE: add a Unicode character browser

Feature or enhancement

IDLE has no way to browse or search for a Unicode character. Inserting one requires already knowing its code point (and typing an escape) or pasting it from another application.

Besides making characters easy to insert, such a tool is useful for learning about Unicode — exploring blocks, looking up a character's name, category, and other properties, and seeing how it is written as an escape, as UTF-8 bytes, as an XML or HTML reference, and in the normalization forms.

I propose a Character Browser, similar to KDE's kcharselect and GNOME Characters, added as a CharSelect extension on the Edit menu (available in both the Shell and the editor).

Features:

• A grid of Unicode blocks selected from a drop-down, scrollable, with hover and selection highlight.
• Search by character name substring, or by code point in many notations: U+2764, 0x2764, ❤, ❤, a bare hex/decimal number, \u/\U/\x/\N{...}/simple escapes, surrogate pairs, and a UTF-8 byte escape run.
• A detail panel with three tabs: an Overview (name, code point, decimal), a Repr tab (literal, Python escape, UTF-8 bytes, surrogate pair, XML and HTML references, \N{...}), and a Unicode tab (block, category, bidi, combining, East Asian width, mirrored, decomposition, numeric values, and normalization forms).
• Insert the character at the cursor of the editor or Shell (replacing any selection, and staying within the Shell's input area), or Copy it to the clipboard. Double-clicking a character in the grid inserts it.
• On the detail tabs, clicking a value copies it and double-clicking inserts it.

When opened with an editor selection, the browser seeds the search with it.

A PR will follow.

Related (not a duplicate): #80648 (help with invisible characters).

Linked PRs

• gh-152945

首先檢查 IDLE 的 Edit 選單中提議的 CharSelect 擴充功能，以及連結的 PR gh-152945。針對 Shell 和編輯器，分別將實作與列出的 Unicode 瀏覽、搜尋、詳細資訊、插入和複製需求進行比較；完成表示已實作並驗證約定的範圍。