Support date object on Date widget

[adongy]

Is your feature request related to a problem? Please describe.
Hi,

netlify-cms supports parsing date objects on some formats (such as TOML).

In that case, multiple formats are supported: either a raw Date object, such as 2020-04-08T09:03:16.371Z, or a simpler date object as defined here:

https://github.com/netlify/netlify-cms/blob/7fcdaef1c92c65d981ac2ebc1077d8da0c3ca285/packages/netlify-cms-core/src/formats/__tests__/frontmatter.spec.js#L84-L93

By default, the date and datetime widgets with no format set will dump the complete Date object.

I'd like to be able to write the shorter YYYY-MM-DD style too.

Describe the solution you'd like
It could be implemented with a new option.

Describe alternatives you've considered
I am unaware of any combination of current options to write a custom format that is unquoted.

Thanks!

[martinjagodic]

Closing as stale and/or outdated.