The code_block command should merge selected paragraphs

[b-kelly]

Describe the bug

Multiple selected paragraphs are transformed into multiple code blocks in rich text mode

To Reproduce

1. Type a line of text, then a second, third, ...
2. Select all three paragraphs and hit the code_block toolbar icon
3. Note that there are three code_blocks

Expected behavior

There should be a single code_block with all three lines included

[tmcentee]

More info on what we need to do here: https://discuss.prosemirror.net/t/how-can-i-merge-paragraphs-in-a-code-block/4542