Add IO::Buffered#flush_on_newline back and set it to true for non-tty

[asterite]

Reverts #7470
Fixes #7431 (somehow)

Before #7470 if you output to the console and redirected that output to somewhere else, for example a file, then you would see output right after each line was emitted.

Once we merged #7470 you would only see output after the IO buffer was full and it was flushed.

That meant that if you did tail -f on that file you wouldn't see new lines appended to it immediately. I believe this is not very intuitive. If you don't output enough data you might think something is broken and lose a lot of time debugging this.

This PR reverts the behavior back to how it was before.

This is slower, but you still have the option to make is faster (if you don't care about not seeing "correct" output using tail -f) by doing:

STDOUT.flush_on_newline = false

If we don't add this method back, the only options you have are:

• fully buffer the output: fast, but not very intuitive
• no buffer: very slow, but intuitive

With flush_on_newline it's not fast but it's also not very slow. So I think this is a good default.

💭 We could also consider having an environment variable, for example CRYSTAL_STDOUT_FLUSH_ON_NEWLINE so you can control, at runtime, how the program behaves (on startup). That means that you can compile your program once and control how it behaves later on. But I didn't include this here. We can discuss it in a different issue if needed, and do it in a separate PR.

Please 👍 if you agree with this change, 👎 if not.

[asterite]

Let's go forward with this. It's been like this for years, and we only had one complaint and then I changed it. And there's always a way to go back to the old behavior (by changing your code).