function isDeclaration(node) {
  return Boolean(
    node.type === 'FunctionDeclaration' ||
      node.type === 'ClassDeclaration' ||
      node.type === 'VariableDeclaration'
  )
}
Check if `node` is a declaration.

@param {Readonly<MaybeNamedClassDeclaration | MaybeNamedFunctionDeclaration | Node>} node
  Node to check.
@returns {node is Declaration | MaybeNamedClassDeclaration | MaybeNamedFunctionDeclaration}
  Whether `node` is a declaration.