Checking the return value of `fgets` isn't good enough to determine
whether the input stream has reached EOF or not.  `fgets` could have run
in to an error and returned NULL.  Additionally, we can't just check the
buffer contents for `\n` because `fgets` could have read the maximum
buffer size (which may not have ended with a `\n`).

I want to use this callback to implement a fix for:

  https://bugs.ruby-lang.org/issues/21188

The purpose of this commit is to fix Bug #21188.  We need to detect when
stdin has run in to an EOF case.  Unfortunately we can't _call_ the eof
function on IO because it will block.

Here is a short script to demonstrate the issue:

```ruby
x = STDIN.gets
puts x
puts x.eof?
```

If you run the script, then type some characters (but _NOT_ a newline),
then hit Ctrl-D twice, it will print the input string.  Unfortunately,
calling `eof?` will try to read from STDIN again causing us to need a
3rd Ctrl-D to exit the program.

Before introducing the EOF callback to Prism, the input loop looked
kind of like this:

```ruby
loop do
  str = STDIN.gets
  process(str)

  if str.nil?
    p :DONE
  end
end
```

Which required 3 Ctrl-D to exit.  If we naively changed it to something
like this:

```ruby
loop do
  str = STDIN.gets
  process(str)

  if str.eof?
    p :DONE
  end
end
```

It would still require 3 Ctrl-D because `eof?` would block.  In this
patch, we're wrapping the IO object, checking the buffer for a newline
and length, and then using that to simulate a non-blocking eof? method.

[Bug #21188]