IO: remove flush_on_newline and only sync on TTY devices

[asterite]

Fixes #7431

Explanation for this here

Merging this or not is still debatable, but the pros I see are:

• remove the strange flush_on_newline property that's actually a hack to be able to see TTY output as it's written, when sync does that in a more unified way (and works too if you call io.print). This also simplifies code because there's less code :-)
• improve performance when you redirect the output to a file

I checked and Logger already flushes the IO when a message is written to it.

[ysbaddaden]

Maybe I'd use a String.build in logger's default formatter, to avoid too many sync writes, but that can wait / be benchmarked to see if there is any real benefit.

[RX14]

This seems to me like a bad idea, since pipes are not TTYs, this will break crystal programs which are run as subprocesses and cause hangs. I suggest making STDIO sync always. If people need more performance they can always turn it off.

[RX14]

I've run across this again, I've written a program which implements the i3bar protocol which streams JSON over STDOUT. It's certainly confusing that the behavior of the program is different when you run it in the terminal and when it's run by another program (and this confused me for a couple of days despite having prior knowledge of this issue).

Does anyone have any comments on my above suggestion or I'll create a PR to make STDIO always sync.

[asterite]

I don't understand why this is a problem in Crystal but not in Ruby.

[RX14]

@asterite I've just looked at ruby and python, they take the approach from this PR: behaving differently based on whether the output is a TTY or not. So it's clearly debatable, I just think that choosing consistency over performance is better. People can always turn off STDOUT.sync = false if they hit a performance bottleneck in their code. I just think that'll be rare.