markdown preview: Improve live preview (#10205)

This PR contains various improvements for the markdown preview (some of
which were originally part of #7601).
Some improvements can be seen in the video (see also release notes down
below):


https://github.com/zed-industries/zed/assets/53836821/93324ee8-d366-464a-9728-981eddbfdaf7

Release Notes:
- Added action to open markdown preview in the same pane
- Added support for displaying channel notes in markdown preview
- Added support for displaying the current active editor when opening
markdown preview
- Added support for scrolling the editor to the corresponding block when
double clicking an element in markdown preview
- Improved pane creation handling when opening markdown preview
- Fixed markdown preview displaying non-markdown files

crates/markdown_preview/src/markdown_parser.rs

@@ -270,7 +270,7 @@ impl<'a> MarkdownParser<'a> {
                             regions.push(ParsedRegion {
                                 code: false,
                                 link: Some(Link::Web {
-                                    url: t[range].to_string(),
+                                    url: link.as_str().to_string(),
                                 }),
                             });