Add support for layout plugins

tagging for findability

A green PHPUnit test suite despite this impressively implementing final class LayoutComponentTest extends ComponentSourceTestBase for final class LayoutComponent extends ComponentSourceBase implements ContainerFactoryPluginInterface, ComponentSourceWithSlotsInterface … 😮👏

Looking forward to reviewing this! 😄

Do you happen to have a screenshot or screencast you'd like to post here? 😇🙏

Thanks for the kind words! I posted a very basic screencast in the XB channel in Drupal slack yesterday of the configuration form working.

It's a webm file which isn't supported by drupal.org apparently, so sharing the link: https://drupal.slack.com/archives/C072JMEPUS1/p1752030986447949

Note for the review that the code is still in a POC state. Functionally it's working well, but there is certainly some cleanup that needs to be done to make the code cleaner.