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.

[crazywhalecc]

@henderkes I really dont' like the current skeleton command. It's a little redundant and not very user-friendly though. At least, I feel I probably won't use it myself just because it's not very helpful. At the same time, I also spent too much unnecessary time on this command.

Do you have any better ideas? Perhaps we could reduce some prompts and leave it to the vendor users to implement themselves, or just replace skeleton command to a clone command (duplicate the current library or extension implementation).