Move tmk_core/common/<plat>

[zvecr]

Description

Migrations:
tmk_core/common/* becomes platforms/*
tmk_core/common/<plat>/ becomes platforms/<plat>/

A few other alignments of terminology:
native.mk aligned to test to match the "tmk_core/common/test" folder - "native" can be for any kmonad style integration.

And with that tmk_core/common is no more!

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).