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.

Read Lib/idlelib/editor.py lines 1365-1385 first, then review issue 136061 and the linked discussion in PR 134874. Clarify whether this issue covers simplifying backspace handling, deprecating tab handling, or both; it is done when the agreed scope is implemented and the obsolete behavior is addressed.