Alacritty: Support changing cursor shape

[topaxi]

Neovim supports changing the cursor shape in supported terminals: https://github.com/neovim/neovim/wiki/FAQ#how-can-i-change-the-cursor-shape-in-the-terminal

This basically means we get a | shape for insert mode, _ shape for replace mode and the default rectangle in command mode.

[CarwynNelson]

+1 for this. I've recently switched to graphical emacs (with evil mode) and this feature is always something I miss when I go back to regular vim.

I'm sure if I were to come back to neovim using alacritty this would be a very nice feature.

[net]

I like to use _ in fish shell instead of █.

[jmacdonald]

Would be great to see general config support for this too, in addition to the escape code-driven approach proposed above!