This patch moves the utility from the app/text_area to os/vfs.h to make
it easier to use by other components. By hosting the 'New_file' as a
friend alongside the 'Directory', we can now pass a 'Directory' as
constructor argument, which is consistent with other utilities such as
'File_content'.
As a further improvement, the new version supports the implicit creation
of the directory hierarchy leading to the new file.
Issue #4032
100583e262