Add YYYY-MM-DD (ISO 8601) date format to presets

[ranidspace]

Description

The YYYY-MM-DD date format is widely used all around the world, and is the one which is used where I'm from. It was a small annoyance to have to manually add it as a custom format every time I logged out and back in, or if I ever set up a new device. It would just save a few seconds here and there for quite a few people.

The YYYY/MM/DD format already exists in Cinny, and I kept it because it's also used, albeit much rarer.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[ranidspace]

I have read the CLA Document and I hereby sign the CLA