vim: Surround in visual mode (#13347)

Adds support for surrounding text in visual/visual-line/visual-block
mode by re-using the `AddSurrounds` operator. There is no default
binding though so the user must follow the instructions to enable it.

Note that the behaviour varies slightly for the visual-line and
visual-block modes. In visual-line mode the surrounds are placed on
separate lines (the vim-surround extension also indents the contents but
I opted not to as that behaviour is less important with the use of code
formatters). In visual-block mode each of the selected regions is
surrounded and the cursor returns to the beginning of the selection
after the action is complete.

Release Notes:

- Added action to surround text in visual mode (no default binding).

Fixes #13122

docs/src/vim.md

@@ -293,6 +293,22 @@ Subword motion is not enabled by default. To enable it, add these bindings to yo
   },
 ```
 
+Surrounding the selection in visual mode is also not enabled by default (`shift-s` normally behaves like `c`). To enable it, add the following to your keymap.
+
+```json
+  {
+    "context": "Editor && vim_mode == visual && !VimWaiting && !VimObject",
+    "bindings": {
+      "shift-s": [
+        "vim::PushOperator",
+        {
+          "AddSurrounds": {}
+        }
+      ]
+    }
+  }
+```
+
 ## Supported plugins
 
 Zed has nascent support for some Vim plugins: