Add UV::S_IFDIR + UV::S_IFREG constants

This PR adds two new constants:

- `UV::S_IFDIR`
- `UV::S_IFREG`

These constants allow differentiation between regular files (S_IFREG) and directories (S_IFDIR) when used for bitwise AND operations against the `"mode"` value returned from `uv_fs_stat/fstat/lstat()`.

Example:

```php
<?php
uv_fs_open(uv_default_loop(), __FILE__, UV::O_RDONLY, 0, function($r){
    uv_fs_fstat(uv_default_loop(), $r, function($result, $da) {
        echo "is directory: ", ($da['mode'] & UV::S_IFDIR ? 'yes' : 'no'), "\n";
        echo "is file: ", ($da['mode'] & UV::S_IFREG ? 'yes' : 'no'), "\n";
    });
});
uv_run();
```