Add DECSCUSR cursor shape changes for vi mode switching

When editorMode is set to vim, the cursor shape doesn't change between normal and insert modes. Most vi-mode implementations (vim, neovim, zsh vi-mode, fish) send DECSCUSR escape sequences to visually distinguish modes:

• Insert mode: blinking bar (\033[5 q) or steady bar (\033[6 q)
• Normal mode: blinking block (\033[1 q) or steady block (\033[2 q)

This is a well-established UX pattern that helps users know which mode they're in at a glance, without relying on a status indicator. It works across most modern terminals (kitty, iTerm2, WezTerm, Alacritty, Windows Terminal, etc.).

Requested behavior: When vi mode is active, send DECSCUSR sequences on mode transitions. Ideally configurable, but even hardcoded block/bar would be a big improvement.