Methods that return a specific range of numbers should have more accurate return types

[ahkhanjani]

E.g. .day() returns a number ranged from 0 to 6, yet the return type is number. It should instead be 0 | 1 | 2 | 3 | 4 | 5 | 6. If this doesn't cause any real issues that I'm unaware of, it can be beneficial in some situations. For example:

const WEEKDAYS = ['Sunday', 'Monday', ..., 'Friday'] as const;

WEEKDAYS[dayjs().day()]; // Currently can be undefined.

With the more accurate return type there will be no need for a runtime check or an exclamation mark.

[ahkhanjani]

Closing since #2630