Allow codegen of `keymap.json` => `keymap.c` without requiring layers/layout etc.

[tzarc]

Description

Allows for keymap.c to coexist with keymap.json. Anything in keymap.json will take precedence and supplying stuff in both will cause failures, but that comes with the territory.

Have migrated the onekey console keymap so that there's at least one keymap in-tree.

Minimal keymap.json could be something like this:

{
    "converter": "promicro_rp2040",
    "config": {
        "features": {
            "unicode": true
        }
    }
}

...with a corresponding keymap.c as per normal usage patterns.

Types of Changes

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

Checklist

• [x] My code follows the code style of this project: C, Python
• [x] 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.
• [x] 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).

[tzarc]

Moved back to draft until I get time to sort out test failures etc. Need to go digging as per Z's comment.

Across the org: https://github.com/search?q=org%3Aqmk+KEYMAP_GOES_HERE&type=code

[tzarc]

Cleared draft state. Seems like it's now properly working both inside qmk_firmware and inside qmk_userspace.

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