Update TTY checks

The function `stream_isatty()` ([PHP manual](https://www.php.net/manual/en/function.stream-isatty.php)) is available since PHP 7.2.0/8.0.

> Determines if stream stream refers to a valid terminal type device. This is a more portable version of posix_isatty(), since it works on Windows systems too.

This update also enables the use of an Output stream as an argument without triggering a warning, as in the folowing code.

```
define( 'STDOUT', fopen( 'php://output', 'w' ) );
var_dump(posix_isatty(STDOUT));
var_dump(stream_isatty(STDOUT));

=>

<b>Warning</b>:  posix_isatty(): could not use stream of type 'Output' in <b>/usr/local/var/www/wp-cli/php/boot-fpm.php</b> on line <b>22</b><br />
bool(false)
bool(false)
```

It is necessary to define `STDOUT` in this way while using the `fpm-fcgi` SAPI. This is useful when running lots of WP-CLI commands in a high-volume task scheduler where up to 95% of CPU time is wasted on the redundant work of loading PHP code. We found that php-fpm's opcode caching reduces resource usage significantly, idling hundreds of CPUs that are otherwise occupied loading the same code over and over again.

The `fpm-fcgi` SAPI runs WP CLI through a modified boot script. A command line program passes commands via an FCGI client and returns results on standard streams. This work will also be contributed to the `wp-cli` project.

Author: skeltoac

lib/cli/Shell.php

@@ -91,7 +91,11 @@ static public function isPiped() {
 		if ($shellPipe !== false) {
 			return filter_var($shellPipe, FILTER_VALIDATE_BOOLEAN);
 		} else {
-			return (function_exists('posix_isatty') && !posix_isatty(STDOUT));
+			if ( function_exists('stream_isatty') ) {
+				return !stream_isatty(STDOUT);
+			} else {
+				return (function_exists('posix_isatty') && !posix_isatty(STDOUT));
+			}
 		}
 	}
 

lib/cli/Streams.php

@@ -14,7 +14,11 @@ static function _call( $func, $args ) {
 	}
 
 	static public function isTty() {
-		return (function_exists('posix_isatty') && posix_isatty(static::$out));
+		if ( function_exists('stream_isatty') ) {
+			return !stream_isatty(static::$out);
+		} else {
+			return (function_exists('posix_isatty') && !posix_isatty(static::$out));
+		}
 	}
 
 	/**