Issue Overview

The idea is to add an has_block() function or similar to check if a post uses a specific block. This would be similar to the has_shortcode() function.

Possible Solution

Implementation could be similar to the gutenberg_content_has_blocks() function:

This function is optimized for speed and simply uses strpos to check for <!-- wp:. has_block() could similarly check for <!-- wp:blockname.

Alternatively, we could load all of the blocks for the post and loop through them until we found a block with that name. Maybe both implementations should be available (different functions or use a flag parameter) for different use cases.