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/sources.rs

@@ -38,7 +38,7 @@ pub(super) fn written(node: &Node, source: &str) -> Option<(Range<usize>, Escape
 }
 
 /// A code span's text, one end space cut each side.
-fn inside(source: &str, whole: Range<usize>, mark: char) -> Range<usize> {
+pub(crate) fn inside(source: &str, whole: Range<usize>, mark: char) -> Range<usize> {
     let written = &source[whole.clone()];
     let open = written.len() - written.trim_start_matches(mark).len();
     let close = written.len() - written.trim_end_matches(mark).len();