Export layer names into json when cli arg is set

[dawosch]

Description

You can define layer names by enum. When you want to convert your keymap.c file to json with the --no-cpp flag the layer names will be used within your keycodes.
But then you don't know what layer name points to what layer.

With the new --export-layer-names the layer names will be exported under the layer_names attribute within the json file.
Then the indices will point to the corresponding layer.

Types of Changes

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

Issues Fixed or Closed by This PR

24570

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).