String Type Unix timestamp support

[lqzhgood]

Describe the bug

dayjs("1564482923186").format('YYYY-MM-DD') ==> "1567-12-29" ❌
dayjs(1564482923186).format('YYYY-MM-DD') ==> "2019-07-30"  ✔️

Expected behavior

dayjs("1564482923186").format('YYYY-MM-DD') ==>  "2019-07-30"  ✔️

Information

• Day.js Version 1.9.3
• OS: Windows
• Browser Chrome 86
• Time zone: UTC+08:00

[iamkun]

a unit timestamp is always a Number not a String

[lqzhgood]

I know that timestamps are numeric, and supporting this will be stronger.

Or for this kind of obvious error, error should be reported directly instead of giving an error value.

like new Date("1564482923186") ==> Invalid Date

###################################
我知道时间戳是数字类型，支持这个会更强壮吧。
或者对于这种 明显 的错误， 直接报错可能比给出一个错误的值更好。
类似于 new Date("1564482923186") ==> Invalid Date

[iamkun]

Ture, we can add this as a special plugin

[iamkun]

Added in #1163