qmd writer drops trailing blank line inside a code block

[rundel]

A CodeBlock with text that ends with \n round-trips with the trailing newline removed. The writer emits the closing fence directly after the last content character, so the re-parser sees one fewer line of content. Could be fixed either in the parser or the writer.

$ printf -- '```markdown\nfoo\n\n```\n'
```markdown
foo

```

$ printf -- '```markdown\nfoo\n\n```\n' | cargo run --bin pampa --
[ CodeBlock ( "" , ["markdown"] , [] ) "foo\n" ]

$ printf -- '```markdown\nfoo\n\n```\n' | cargo run --bin pampa -- -t qmd
```markdown
foo
```

$ printf -- '```markdown\nfoo\n\n```\n' | cargo run --bin pampa -- -t qmd | cargo run --bin pampa --
[ CodeBlock ( "" , ["markdown"] , [] ) "foo" ]

Examples from quarto-web:

• https://github.com/quarto-dev/quarto-web/blob/8b1be24df1c9301f2700c77e41e7a9f789535717/docs/authoring/includes.qmd#L35
• https://github.com/quarto-dev/quarto-web/blob/8b1be24df1c9301f2700c77e41e7a9f789535717/docs/authoring/cross-references.qmd#L93
• https://github.com/quarto-dev/quarto-web/blob/8b1be24df1c9301f2700c77e41e7a9f789535717/docs/blog/posts/2024-07-11-1.5-release/index.qmd#L24