Generate keymap dd keycodes to header

[zvecr]

Description

Now keymap.h is unused, we can re-purpose it for a sibling to keymap.c when processing a keymap.json.

This will allow additional files to include the header, and implement with process_record_user.

In the future this header can be reused for more user facing items like tap dance, unicode, etc.

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).