Horizontal rule is not encoded as --- when converting to markdown #425

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