Add scrolling functionality to markdown preview mode (#44585)

Closes #21324

Adds four new commands:
- `markdown::MoveUp`, `markdown::MoveDown` - these scroll up and down in
markdown preview mode, by no more than the height of a large headline.

- `markdown::MoveUpByItem`, and `markdown::MoveDownByItem` - these
scroll up and down by the height of the item at the top of the markdown
preview window. So headlines and large codeblocks, for instance, scroll
further than individual paragraph lines.

Also attempts to create sensible defaults:
`down` -> `markdown::ScrollDown`
`up` -> `markdown::ScrollUp`
`alt-down` -> `markdown::ScrollDownByItem`
`alt-up` -> `markdown::ScrollUpByItem`

And in Vim:

`ctrl-u` -> `markdown::ScrollPageUp`
`ctrl-d` -> `markdown::ScrollPageDown`
`ctrl-e` -> `markdown::ScrollDown`
`ctrl-y` -> `markdown::ScrollUp`


Release Notes:

- Added commands `markdown::ScrollUp`, `markdown::ScrollDown`,
`markdown::ScrollUpByItem`, and `markdown::ScrollDownByItem`
- Changed commands `markdown::MovePageUp` to `markdown::ScrollPageUp`
and `markdown::MovePageDown` to `markdown::ScrollPageDown`

crates/vim/src/test/vim_test_context.rs

@@ -28,6 +28,7 @@ impl VimTestContext {
             search::init(cx);
             theme::init(theme::LoadThemes::JustBase, cx);
             settings_ui::init(cx);
+            markdown_preview::init(cx);
         });
     }