Upgrade xterm.js to 5.2.1

[isontheline]

Feature description

• [x] https://github.com/xtermjs/xterm.js/releases/tag/5.2.0
  • [x] 5.2.1 : https://github.com/xtermjs/xterm.js/releases/tag/5.2.1
• [x] https://github.com/xtermjs/xterm.js/issues/4474
  • [x] https://github.com/isontheline/pro.webssh.net/blob/bbb4a2eb9014971c084b48907c40c8f6bd9e6d0c/src/ssh/assets/xterm-webssh.js#L327
• [x] https://github.com/isontheline/pro.webssh.net/issues/936
• [ ] https://github.com/isontheline/pro.webssh.net/issues/932
  • [x] https://github.com/xtermjs/xterm.js/issues/4579