Horizontal rule is not encoded as --- when converting to markdown

[kane-knowby]

Steps to Reproduce

1. Add a horizontal rule using the rich text editor
2. Convert your document to markdown using ParchmentMarkdownCodec().encode(document)
3. Notice the horizontal rule is not present at all
4. Expected value would be '---'

Environment

OS: iOS/Web/Android
Flutter version: 3.24
Fleather version: 1.18.0

[amantoux]

Thank you for reporting this @kane-knowby
We will address this with #418