htaccess functions should be a service

Problem/Motivation

As part of the general clean-up of the File API, file_save_htaccess() and file_ensure_htaccess() need to be deprecated and moved into services.

Proposed Resolution

Create a new service 'file.htaccess_writer' and deprecate file_ensure_htaccess() in favour of \Drupal\Core\File\HtaccessWriterInterface::ensure()

Deprecate file_save_htaccess() in favour of the FileSecurity component added in #3013210: Add drupal/core-filesecurity component for writing htaccess files