Provide autosuggestions

[aca]

Provide fish like autosuggestion. By default it suggest from history. But can be customized (maybe use it to provide AI suggestions?)

# enabled by default
set edit:auto-suggestion:enabled = $true

# optional
set edit:auto-suggestion:provider = {|code|
  if (str:has-prefix $code "hello ") {
    put "world"
  }
}

# default keymap
set edit:insert:binding[Ctrl-F] = $edit:auto-suggestion:accept~

fixes #322

[aca]