Blocks: configurable storage format

One reason why we are trying to avoid blocks in our projects is the JSON storage format which makes it hard to manipulate content via simple search and replace actions. This is way easier with YAML data.

With the new Symfony YAML handler landing as default in Kirby 6, it would be great if the blocks field would allow for storage in YAML (again). This could either be a field option (format: yaml) or a site configuration.