[Super Editor] - Code block newline insertion position is wrong

[matthew-carroll]

Repro:

• Create a code block with at least a few characters of content
• Place the caret in the middle of the text
• Insert a newline

Actual result:
The newline is inserted at the end of the code block

Expected result:
The newline should split the code block into two blocks at the caret position.

The root problem is in the code block newline insertion command where we should be using the caret node position, but instead we're using the end of the code block.