Improved code block display in Markdown editing mode

### What
When in Markdown editing mode, code blocks don't have any sort of formatting or highlighting.


### Why
It would make reading Markdown notes in editing mode easier if we formatted code blocks similarly to how we format code snippets (text wrapped in single backticks).

### How
It would be nice if, at a minimum, we formatted code blocks similarly to how we format single-line snippets.


Start in the Markdown editing-mode entry point and trace how single-backtick code snippets are formatted. Compare that behavior with fenced code blocks and verify in the macOS editor that code blocks have equivalent visual formatting.

Written by the indexing model from the issue text.