[php-src] Issue #20783: Add `\f` as a whitespace character in `trim`

Issue: https://github.com/php/php-src/issues/20783
Author: LamentXU123

### Description

Hi.
The `trim` function will remove whitespaces in the parameters. The default characters are ` \n\r\t\v\0`, [source](https://github.com/php/php-src/blob/85b681a2061cfa5db68d86f8101d331a3af40da0/ext/standard/string.c#L524).
I think maybe `\f` is missing here? In most of the cases in php (such as `is_numeric`, [source](https://github.com/php/php-src/blob/85b681a2061cfa5db68d86f8101d331a3af40da0/Zend/zend_operators.c#L3643)) we recognized `\f` as a whitespace. Also in libc `isspace.c` we too include `\f` as whitespaces:
```
/*-
 * See the file LICENSE for redistribution information.
 *
 * Copyright (c) 2005,2007 Oracle.  All rights reserved.
 *
 * $Id: myisspace.c,v 1.4 2007/05/17 15:14:54 bostic Exp $
 */

 /*
  * myisspace --
  *
  * PUBLIC: #ifndef HAVE_ISSPACE
  * PUBLIC: int myisspace __P((int));
  * PUBLIC: #endif
  */
int
myisspace(c)
int c;
{
	return (c == '\t' || c == '\n' ||
		c == '\v' || c == '\f' || c == '\r' || c == ' ' ? 1 : 0);
}
```
Since the [document](https://www.php.net/manual/en/function.trim.php) indicate that `trim` would "Strip whitespace (or other characters) from the beginning and end of a string", I think `\f` maybe an unexpected case here. Thanks.

### PHP Version

```plain
all version
```

### Operating System

_No response_