Code block constraint and normalization

[cloverich]

• When selecting text across multiple paragraphs, converting to code block converts it into multiple code blocks. Notion also does this and it drives me insane. This is likely a fix that can be done in plate. update: Moving this to a separate issue as it affects more than code blocks
• Prevent nested code blocks. I think this is also reported in plate

[cloverich]

Original description is only partially accurate as the libraries were upgraded and re-set. See linked issues and especially #176 for more context.

[cloverich]

There's a bug where the initial code_block values aren't normalized correctly, and modifying the text will crash the editor; reloading the page fixes so the slate->mdast->slate transformation fixes. Probably how the code lines are generated.

[cloverich]

Also, the code_block width growing too large causes the parent application window to gain an x-scroll; instead it should have its max-width to the editor and itself handle scrolling