vim fixes for find&replace (#2995)

* allow replacing with the empty string to delete
* fix <enter> for ReplaceNext (in vim mode)

Release Notes:

- allow replacement to be empty

assets/keymaps/vim.json

@@ -531,7 +531,7 @@
     }
   },
   {
-    "context": "BufferSearchBar > VimEnabled",
+    "context": "BufferSearchBar && !in_replace > VimEnabled",
     "bindings": {
       "enter": "vim::SearchSubmit",
       "escape": "buffer_search::Dismiss"