fix how class names from core get passed through register block extension

[fabiankaegy]

Description of the Change

When filtering the editor.BlockListBlock to add a custom class name to it we need to ensure we correctly pipe through any already existing classes. This was done incorrectly. You can see an example of how core does it here: Here is an example of how it is done in core: https://github.com/WordPress/gutenberg/blob/70ba9218ae53e31d2c26689eed2e698dcea7f3aa/packages/block-editor/src/hooks/layout.js#L457-L460

How to test the Change

All tests should pass

Changelog Entry

Fixed - Make sure any class names which get added via editor.BlockListBlock filter get passed through correctly

Credits

Props @fabiankaegy

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[github-actions]

Size Change: +12 B (0%)

Total Size: 38.3 kB

Filename	Size	Change
dist/index.js	38.3 kB	+12 B (0%)

_{compressed-size-action}

[Antonio-Laguna]

LGTM!