fix(code-blocks): press Enter at start of first code block to create paragraph

[alexwarren]

Closes #197

Describe your changes

This PR make it so that if a code block is the first thing in a document, then when you press Enter at the start of that code block, a paragraph is created above.

This only applies if you're at the start of the code block, and only if that code block is the first thing in the doc.

Screen.Recording.2025-02-28.at.11.10.40.mov

PR Checklist

• All new/changed functionality includes unit and (optionally) e2e tests as appropriate
• All new/changed functions have /** ... */ docs
• I've added the bug/enhancement and other labels as appropriate

Environment(s) tested

Chrome, Safari

[dancormier]

Nice work @alexwarren! I looked through the code seems solid. Only once minor suggested change on my part. Once that's resolved, I'm good to see this merged. Thanks!

Out of scope (no changes expected)

I start to get a little scared when I see files with more than a couple hundred lines. Long term, I think we'd be wise to separate src/rich-text/commands/index.ts and the related test file into per-function files. This would probably make maintenance and modification easier.