Make components from ancestor themes available

Problem/Motivation

#3529067: Make Island plugins configurable will make this configurable, but we need a sensible default now.

ComponentLibraryPanel::getDefinitionsForProvider() is excluding component from any theme other than the default one. That's not good because Project often use a nearly empty custom theme overriding a shared theme implementing a design ssytem

Proposed resolution

Simpler solution, just remove this:

      if ($definition['extension_type']->value === 'theme' && $definition['provider'] !== $provider) {
        continue;
      }

Better solution: get ancestor themes of the default active theme.