In vim, the current buffer can be closed with :bdelete (or :bd or
:bdel). That looks what the quit command does right now.

The behaviour doesn't exactly match vim's when buffers contain unsaved
modifications. Vim will ask you to save before :bd or to add '!' at the
end to close the buffer without saving the modifications.

I'm not entirely convinced builder needs to match this behaviour
exactly, having :bd doing something useful is a nice start in any case,
I believe.