gh-158955: PyREPL Update changed line planning for WSL - #158995

PyREPL treated every single-character addition as an insertion, emitting the terminal’s ICH/INS control sequence before writing the character. On WSL, emitting INS at the end of a line could disturb terminal rendering and leave the displayed prompt/cursor out of sync.

This change uses a normal character write when appending at the end of the line, while preserving INS for true mid-line insertions.

Example: typing e after nam| now directly produces name|. INS remains used when inserting into nme to produce name.

• Issue: PyREPL corrupts attribute completion input on WSL/Windows Terminal #158955