builtin: implement unbuffer_stdout/0

[spytheman]

This API addresses this Discord discussion: https://discord.com/channels/592103645835821068/592294828432424960/1161031983506411533

With it, you can implement a simple progress bar, using code like this:

import time
unbuffer_stdout()
for _ in 0 .. 10 {
	print('*')
	time.sleep(100 * time.millisecond)
}
println(' done')

Note that each * will appear in your terminal right away. If you comment the unbuffer_stdout() call, then each star will be buffered, and you will see the whole bar, only at the end (after the println() call).