Only parse front matter at the top of the document

[UnknownPlatypus]

Fixes #244

This is a minimal fix but I'm wondering if we should not do the frontmatter check in parse_root instead since frontmatter is only expected at the start of the file ?

We currently pay (and were paying) the frontmatter check on every - in the document

Maybe something like that

pub fn parse_root(&mut self) -> PResult<Root<'s>> {
    let mut children = vec![];

    if matches!(
        self.language,
        Language::Html
            | Language::Astro
            | Language::Django
            | Language::Jinja
            | Language::Vento
            | Language::Mustache
    ) && self.source.starts_with("---")
    {
        let (kind, raw) = self.with_taken(|parser| {
            parser.parse_front_matter().map(NodeKind::FrontMatter)
        })?;
        children.push(Node { kind, raw });
    }

    while self.chars.peek().is_some() {
        children.push(self.parse_node()?);
    }

    Ok(Root { children })
}

• The pull request description was written manually by me and was not generated by an AI tool or agent.
• I have read and understood the relevant parts of the existing codebase before making these changes.
• I have carefully reviewed and understood all code changes, and ensured that they match the style, architecture, and conventions of the existing codebase.
• I understand that pull requests containing blindly generated or unreviewed AI-generated code may be closed without review.

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