Modify Gutenberg to allow better paste handling

There is a custom paragraph block which gives us limited control over paste handling. It's not ideal to duplicate a core block, and it doesn't retain white spaces. It would be better if we could modify Gutenberg itself so that a filter (or some other method) could be added that allows client apps to customise the paste handling in a more robust way.

Start by examining the custom paragraph block and Gutenberg's existing paste-handling path to understand where whitespace is lost and how client apps currently customize it. Done means client apps can robustly customize paste handling without duplicating a core block, while preserving whitespace.

Written by the indexing model from the issue text.