Add more completion types to `complete-command`

[ficd0]

The complete-command command allows for the following completion types:

• file
• client
• buffer
• command
• shell
• shell-script
• shell-script-candidates

However, there are a few other "types" that I feel would be helpful:

• scope
  • This would just be local, window, buffer, global. However it's
    common enough that providing a type for it and saving the need to spawn a
    shell could be helpful.
• option
  • Would complete from all available (non-hidden) options, the same as the
    completion for %opt{
• value
  • Would complete from all available values, the same as the completion for
    %val{