ts类型别名

语法：

type 别名=类型

let 变量：别名=值

type TypeItem =string | number
let x:TypeItem
x=11
x='你好'

type arrType =string
left arr:arrType =['哈’，’哈‘]

type arrsType =Array<nubmer>
let arrs:arrsType =[11,22,33,44]