3.0: skeleton - package config and code generator

[crazywhalecc]

What does this PR do?

Add skeleton command.

Checklist before merging

If your PR involves the changes mentioned below and completed the action, please tick the corresponding option. If a modification is not involved, please skip it directly.

• If you modified *.php or *.json, run them locally to ensure your changes are valid:
  • [ ] composer cs-fix
  • [ ] composer analyse
  • [ ] composer test
  • [ ] bin/spc dev:sort-config
• If it's an extension or dependency update, please ensure the following:
  • [ ] Add your test combination to src/globals/test-extensions.php.
  • [ ] If adding new or fixing bugs, add commit message containing extension test or test extensions to trigger full test suite.