documents: lists go on, block lines shift, tables change, matches show, fences color by language

The document carries Markdown's editing tools for ElyMark (MARK-06):
Enter continues a list item or ends an empty one, Tab and Shift-Tab
shift an open block's lines, table rows and columns come and go as
one undo step, a match shows in any mode, replacements are one step,
and a fenced block colors by its language.

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

src/editor/mod.rs

@@ -48,7 +48,7 @@ pub use find::{FindOptions, FindWidget, find_all};
 pub use hints::{CompletionItem, CompletionMenu, Signature, SignatureHelp, completions};
 pub use hover::HoverInfo;
 pub use keybindings::{KeySource, Keybinding, KeybindingsEditor, conflicts};
-pub(crate) use keys::bind_keys;
+pub(crate) use keys::{Newline, bind_keys};
 pub(crate) use layout::stack;
 pub use preview::ThemePreview;
 pub use problems::{Problem, ProblemsPanel};