Switch to tracking linewrap on the last cell in a line

This allows us to have newline not affect the wrap status of a line.

Now a lines wrapping status is changed only when the last cell
in the line is changed. This actually matches the behavior of many other
terminal emulators so is probably a good thing from a ecosystem
compatibility perspective.

The fish shell expects this weird behavior of newline not changing
wrapping status, for unknown reasons, which is the actual motivation for
doing all this work.

Fixes #5766

kitty/window.py

@@ -278,8 +278,6 @@ def as_text(
         h: List[str] = [pht] if pht else []
         screen.as_text_for_history_buf(h.append, as_ansi, add_wrap_markers)
         if h:
-            if not screen.linebuf.is_continued(0):
-                h[-1] += '\n'
             if as_ansi:
                 h[-1] += '\x1b[m'
         ans = ''.join(chain(h, lines))