fix: daysJS.extend returns typeof dayJS, not an instance

[JacobLey]

Title pretty much says it all. Refer to source code: https://github.com/iamkun/dayjs/blob/dev/src/index.js#L390

Proper typing enables extend chaining in typescript

e.g. dayjs.extend(foo).extend(bar)

Should result in a "patch" update. Technically type change would be a major, but considering the current is incorrect, it is unreasonable to assume anyone is using it as-typed and would therefore have a broken process.