Add IO#tee_from method in backend

[noteflakes]

An basic implementation of the tee command, based on this:

require 'polyphony'

file_path = ARGV[0]

File.open(file_path, 'w+') do |f|
  len = STDOUT.tee_from(STDIN, 2**16)
  f.splice_from(STDIN, len)
end