Set default config_sync_directory outside the webroot

Problem/Motivation

We have a config/sync directory outside the root but currently it has to be set manually every time we set up the project.

Steps to reproduce

Set up the project. After setup, the config sync directory is something like:
$settings['config_sync_directory'] = 'sites/default/files/config_P0mmr3X3zO6cWP8OO1FywKNhkFdgEjK-C-g9KVNUJPYVL2YykVpdsY93yplpe-nCDYpPH9WoAg/sync';

Proposed resolution

Create a settings.php in an assets directory and use composer scaffolding to scaffold settings.php with our custom version which has the config_sync_directory set to ../config/sync.