Add ISO layout for VIA

[Mhilkos]

Description

Added ISO keyboard layout for Keychron V6 Max to be used on VIA as a JSON file.
(I later realized that this is in the keychron website, but not in the repository. It would be nice if the maintainer of this branch could either add my commit or the one in the website to the repo, Thanks 👍🏻 )

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Issues Fixed or Closed by This PR

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[github-actions]

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