function isValidModifier(modifierNode, customModifiers) {
  const modifier = modifierNode.name
  return (
    // built-in aliases
    VALID_MODIFIERS.has(modifier) ||
    // keyCode
    Number.isInteger(Number.parseInt(modifier, 10)) ||
    // keyAlias (an Unicode character)
    [...modifier].length === 1 ||
    // keyAlias (special keys)
    KEY_ALIASES.has(modifier) ||
    // custom modifiers
    customModifiers.has(modifier)
  )
}
@param {VIdentifier} modifierNode
@param {Set<string>} customModifiers