unconfirmed text of CJK IME collapse to one character in Integrated Terminal

The same root cause as https://github.com/Microsoft/vscode/issues/24557 and we need to fix this in Integrated Terminal (xterm.js) as well. We need to adopt next version of xtermjs once https://github.com/sourcelair/xterm.js/pull/632 is merged.