Allow typing newline with <S-Enter> and enable keyboard enhancement protocol

[ulyssa]

This fixes #271 (and #255 in the right terminals) by enabling the Kitty keyboard enhancement protocol. In order to benefit from this, you'll need to be using a terminal that supports one of:

• The Kitty keyboard enhancement protocol :
  • alacritty
  • foot
  • kitty
  • rio
  • wezterm
• Enabling the key sequence for the xterm-style modifyOtherKeys:
  • xterm
  • mlterm
  • iTerm2 (requires enabling Profiles > Keys > xterm control sequence can enable modifyOtherKeys mode)
  • Possibly others, but tracking down who does or doesn't have support for this one is harder than I would have thought!
• A terminal that uses WinAPI to send keypresses
  • Windows Terminal
  • ConEmu

I'd like to put together a Terminal Recommendations page on the website to help people find terminals with support for image previews and improved keypress support. I'll probably include all of the above there in a table, with columns to show what they support.

Terminals I tried that don't support any way to represent Shift + Enter:

• deepin-terminal
• Eterm (also seems unable to handle unicode)
• gnome-terminal (tracking issue)
• konsole (tracking issue)
• lxterminal
• mate-terminal
• qterminal
• st
• sakura
• Terminal.app
• terminology (cursor is shown in the wrong place for some reason?)
• urxvt
• xfce4-terminal
• yakuake
• zutty
• WSL in Windows Terminal (tracking issue)

deepin-terminal, konsole, qterminal, and yakuake generate SS3 M when you press Shift + Enter, which crossterm doesn't currently parse, but from what I can tell should be parsed as a numpad Enter.