Make C-S work in vim again

Tips from [zsh-vimto](https://github.com/laurenkt/zsh-vimto).

* Remove delay when changing mode
* Enable backspace when returning to viins from vicmd mode

zshenv

@@ -8,6 +8,9 @@ HISTFILE=$HOME/.cache/zsh/histfile
 HISTSIZE=20000
 SAVEHIST=20000
 
+# Remove vi mode switch delay
+KEYTIMEOUT=1
+
 # Enable time stats for long lasting commands
 REPORTTIME=5
 

zshrc

@@ -56,7 +56,7 @@ setopt ignore_eof
 setopt nobeep
 
 # Disable tty flow control, allows vim to use '<Ctrl>S'
-setopt noflowcontrol
+setopt noflowcontrol; stty -ixon
 
 # Enable completions in the middle of a word
 setopt completeinword
@@ -86,6 +86,9 @@ prompt fresh
 # Enable vi mode line editor keymap
 bindkey -v
 
+# Enable backspace after returning to viins from vicmd mode
+bindkey '^?' backward-delete-char
+
 # Enable yank, change, and delete whole line with 'Y', 'cc', and 'dd'
 bindkey -M vicmd 'Y' vi-yank-whole-line