documents: inline formats toggle as Typora's

MarkdownDocument::format toggles bold, italic, code, strike, underline,
link and image at the selection, or clears every inline mark from the
blocks it touches, following Typora's toggleStyle: off inside the style,
wrap a selection, take the caret's word or write an empty pair. Code,
math, HTML and front matter refuse, as does a selection crossing a
block, a link or another mark. An open block keeps its field open.

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

src/documents/tree/mod.rs

@@ -14,7 +14,7 @@ mod stack;
 pub use blocks::{AlertKind, DocumentHeading, alert, is_toc};
 use html_block::html_block_text;
 pub use sources::Escapes;
-pub(crate) use sources::lines_at;
+pub(crate) use sources::{inside, lines_at};
 use stack::close;
 
 /// How a stretch of inline text prints.