Add size() to both FileInputStream and FileOutputStream

[thekid]

Example:

use io\streams\{FileInputStream, FileOutputStream};

$in= new FileInputStream('README.md');
$in->size(); // Size of README.md on disk

$out= new FileOutputStream('test.json');
$out->write(/* ... */);
$out->size(); // Size of file after writing

[thekid]

Released in https://github.com/xp-framework/core/releases/tag/v12.6.0