$LIMA_HOME/_config/defaults.yaml not created by default

### Description

Default lima _config looks like:

```
% tree ~/.lima/_config
/Users/nsoffer/.lima/_config
├── networks.yaml
├── user
└── user.pub
```

With this the defaults feature `_config/default.yaml` has no visibility:
- user doe now know that there can be such file
- user has no way to tell what are the defaults

This easy to fix by creating a defaults file when creating the _config directory.

Questions:
- Who owns the default file? should lima change the defaults file when a new version use new default values?
- The canonical way to solve this on Linux is the defaults come from application/vendor/distro location and user does not own them. User defaults are installed in user own files and override the system defaults (e.g. conf.d/50-user.conf).

If the defaults.yaml is owned by the user it is fine to not create it, but there should be another defaults.yaml not owned by the user that can be copied to or use as a reference when creating user defaults.