Toggle Markdown Preview with the Existing Preview Shortcut
 
## Context

Markdown files already expose actions for opening a rendered preview in
the current pane or to the side. The current-pane preview action focuses
the preview, but there was no matching keyboard path from the preview
back to the source editor.

Users expect the existing preview shortcut to behave like a mode toggle:
press it from the editor to preview, then press it again from the
preview to return to editing. The existing preview view keeps a handle
to its source editor, so the preview can return focus to that editor
without adding a broader workspace-level toggle.

## Feature

Add a markdown-preview-scoped action,
`markdown::CloseAndReturnToEditor`, and bind the existing preview
shortcut to complementary actions:

- In a markdown editor, `ctrl-shift-v` on Linux and Windows, or
`cmd-shift-v` on macOS, dispatches `markdown::OpenPreview`.
- In a markdown preview, the same platform shortcut dispatches
`markdown::CloseAndReturnToEditor`.

The close action first activates and focuses the source editor. If the
source editor entity is no longer open in any pane, the action adds it
back to the active pane before closing the preview. The preview is
closed with `SaveIntent::Skip` because the preview is not the owner of
source-buffer changes.

Existing markdown preview shortcuts remain available.

## Alternatives

- Bind `escape` to close the preview. This is easy to discover, but it
does not provide a symmetric editor/preview toggle and can overlap with
transient UI such as search.
- Add a new dedicated toggle chord, such as `ctrl-alt-v` / `cmd-alt-v`.
This avoids changing the preview shortcut's meaning in the preview
context, but it makes users learn a second shortcut for the same
preview/edit workflow.
- Teach `markdown::OpenPreview` to toggle based on the active item. That
would mix editor-scoped preview creation with preview-scoped close
behavior and make the action depend on more workspace state.

## Consequences

- The toggle behavior is implemented by context-specific keybindings
rather than one action that changes meaning globally.
- Returning to edit mode works even when the original editor tab has
been closed while the preview remains open.
- The preview close path has focused GPUI tests because it relies on
pane activation, focus restoration, and asynchronous item closing.

Release Notes:

- Add a markdown action to close the current preview and focus the
related source file