Weekday

Weekday 添加了 `.weekday()` API 来获取或设置区域设置感知的一周中的某一天。

var weekday = require('dayjs/plugin/weekday')
dayjs.extend(weekday)

// when Sunday is the first day of the week
dayjs().weekday(-7); // last Sunday
dayjs().weekday(7); // next Sunday

// when Monday is the first day of the week
dayjs().weekday(-7) // last Monday
dayjs().weekday(7) // next Monday