svg_preview: Ensure preview properly updates in follow mode (#38081)

This fixes an issue where we would not update neither the path nor the
editor that was listened to during follow mode, which in turn would
cause the preview to become stale.

Fix here is to update the subscription whenever the active item changes
and also update the associated path accordingly.


Release Notes:

- Fixed an issue where the SVG preview would not update when following
the active editor.

Cargo.lock

@@ -15922,11 +15922,10 @@ dependencies = [
  "editor",
  "file_icons",
  "gpui",
- "project",
+ "multi_buffer",
  "ui",
  "workspace",
  "workspace-hack",
- "worktree",
 ]
 
 [[package]]