MarkdownDocument: a Markdown file in one CodeEditor, shown visual, source, split or preview

Visual draws the top-level blocks in a gpui list; a pressed block opens as
its source and its typing is the text at once, written back as one undo
step. CodeEditor takes the owner's line colors, resets with a fresh
history and keeps at most 64 MiB of undo text. MarkdownMode gains Preview.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/documents/markdown.rs

@@ -23,6 +23,13 @@ pub(crate) fn tree(text: &str) -> mdast::Node {
     markdown::to_mdast(text, &options()).expect("markdown without MDX always parses")
 }
 
+/// A whole file as a syntax tree: front matter too.
+pub(crate) fn file_tree(text: &str) -> mdast::Node {
+    let mut options = options();
+    options.constructs.frontmatter = true;
+    markdown::to_mdast(text, &options).expect("markdown without MDX always parses")
+}
+
 /// The bytes of the source a parsed node spans.
 pub(crate) fn span(node: &mdast::Node) -> Range<usize> {
     let at = node.position().expect("a parsed node has its position");