fix: read front matter at start only

pulldown-cmark's metadata flags take `---`, a line, and `---` as front
matter anywhere, even inside a list item, and mdtidy then wrote it at
the left margin, outside the item. Front matter is now found only at
the very start, with Prettier's rule: `---` or `+++` and an optional
language, closed by the same delimiter, or by `...` for YAML. The rest
of the document is parsed without the metadata flags.