Modernize Code In IDLELib Editor Backspace

(and potentially deprecate handling of tabs in IDLE, though likely separate issue)

In https://github.com/python/cpython/issues/136061, https://github.com/python/cpython/blob/561212a03306cb2f246771e9c4748bec1dc41405/Lib/idlelib/editor.py#L1365-L1385 is mentioned as an area of code with quadratic complexity, which may lead to performance issues; though this is unlikely to be of large impact in practice because attackers with the ability to get a huge file loaded and/or modify configuration files to change the tab width can do much worse things than getting IDLE to hang on hitting backspace.

However, the complexity there comes from a loop which handles situations (files with tabs) that either cannot or should not occur; in the current default IDLE, you can't put tabs, and a space after a tab should just be deleted in a file coming from elsewhere as in indicated in https://github.com/python/cpython/pull/134874#issuecomment-3014981765. Therefore, simplifying the function specification and/or deprecating IDLE to handle tabs (which we should do) will allow elimination of this piece of obsolete code, as indicated in https://github.com/python/cpython/issues/136061#issuecomment-3016093746.

先閱讀 Lib/idlelib/editor.py 的第 1365–1385 行，然後檢視 issue 136061 以及 PR 134874 中連結的討論。釐清此 issue 涵蓋的是簡化 Backspace 處理、淘汰 Tab 處理，還是兩者皆包括；在約定的範圍完成實作並處理過時的行為後，此 issue 即告完成。