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/editor/mod.rs

@@ -19,6 +19,7 @@ mod line;
 mod moves;
 mod preview;
 mod problems;
+mod prose;
 mod refactor;
 mod row;
 mod search;