Rich-text mode: add empty element after code blocks

[hamvocke]

When inserting a code block in rich-text mode at the end of the document, it's impossible to move the cursor below the code block. If I wanted to continue writing text after the code block, I can't just put my cursor below the code block (e.g. via mouse click or arrow key navigation) which can be quite jarring. Currently, the only remedy is to hit Mod-Enter within the code block to exit it and insert a newline right after.

For Tables, we do something similar to what I'm proposing here: If there's no subsequent node, we make sure to insert an empty paragraph right after the to-be-inserted table. When inserting commands, we should be able to do the same quite easily. Most likely this is a matter of chaining the "insert code block" command with the "insert empty paragraph if required" command.