bug fix: HR vs YAML block parsing in breakQuartoMd

[gordonwoodhull]

I'm not sure if we should fix this, but I wanted to submit it for discussion.

There is a difference between how Pandoc and breakQuartoMd parse horizontal rules versus YAML blocks with

---

Pandoc "strongly recommends" that you put blank lines before and after an HR, but only checks for the latter.

OTOH breakQuartoMd only parses --- as an HR if it is both preceded and followed by a blank line. Otherwise it might be starting a YAML block (if it finds the end of one).

This change tells breakQuartoMd to treat --- as an HR if it followed by a blank line, like Pandoc. It no longer looks at line before.

The Problem

The quarto-marimo index.qmd has a construction that looks like:

---
title: marimo + quarto
format: html
---

# Heading
---

Some content here (including code blocks).

---

More content.

and now that it relies on breakQuartoMd, it loses some code blocks and fails.

I'm completely fine with telling them to add a newline, but thought I'd raise this fix for consideration.

Blast radius

The function gets a lot of use, so it's little scary to change it

Components that depend on breakQuartoMd

1. Project Code Cell Resolution - projectResolveCodeCellsForFile for understanding project structure
2. Markdown Include Expansion - expandIncludes for processing {{< include >}} directives
3. Language Cell Handling - handleLanguageCells for processing different language cells
4. Jupyter Notebook Conversion - quartoMdToJupyter for converting qmd to ipynb
5. OJS Resource Extraction - extractResolvedResourceFilenamesFromQmd for Observable JS
6. YAML Intelligence - automationFromGoodParseMarkdown for editor completions/validations

But the change does not affect the parsing of actual YAML block openings, since those would always have content in the next line.

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