Add converter support to keymap.json

[zvecr]

Description

Building on the recent bug fixes, its now possible to add in support for CONVERT_TO. Eventually this could power a drop down somewhere on configurator.

Potentially it could extend the existing config object, and reuse mappings/info_rules.json...?

Key (and potentially any parents) up for grabs still.

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).