Adds a simplified version of Twenty Twenty-Three theme that helps testing Block Hooks integration. The theme contains:
- The required index.html template.
- The optional single.html template used with tests.
- 3 template parts where two of them reference patterns.
- 3 patterns referenced in the templates and the template parts.
New tests automatically register 4 custom blocks with the test theme where each of them hooks into another block using all four target relative positions: `before`, `after`, `firstChild`, `lastChild`.
The tests verify that the block gets hooked into the correct positions when targeting:
- template
- template part
- pattern
Props ockham, costdev.
See #59313, #59383.
Follow-up [56610].
git-svn-id: https://develop.svn.wordpress.org/trunk@56759 602fd350-edb4-49c9-b593-d223f7449a82
