Shortcut key for selecting a word and extending the selection in VS Code

i use alt+s for editor.action.smartSelect.grow.

{
  "key": "alt+s",
  "command": "editor.action.smartSelect.grow",
  "when": "editorTextFocus"
}

The shrink/expand selection commands should be what you are looking for. The command names are editor.action.smartSelect.grow (default keybinding shift+alt+right) and editor.action.smartSelect.shrink (default keybinding shift+alt+left).