helix: Allow yank without a selection (#35612)

Related https://github.com/zed-industries/zed/issues/4642

Release Notes:
- Helix: without active selection, pressing `y` in helix mode will yank
a single character under cursor.

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Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

assets/keymaps/vim.json

@@ -390,7 +390,7 @@
       "right": "vim::WrappingRight",
       "h": "vim::WrappingLeft",
       "l": "vim::WrappingRight",
-      "y": "editor::Copy",
+      "y": "vim::HelixYank",
       "alt-;": "vim::OtherEnd",
       "ctrl-r": "vim::Redo",
       "f": ["vim::PushFindForward", { "before": false, "multiline": true }],