feat(date): addYear, addMonth, addDate, addHours, addMinutes, addSeconds, addMilliseconds

[samuelscheit]

adds the following functions (addYear, addMonth, addDate, addHours, addMinutes, addSeconds, addMilliseconds) to the date class.

This makes it easier to work with dates, because to increase the month you would have to write

const date = new Date()
d.setMonth(d.getMonth() + 1)

which will be simplified as

const date = new Date()
d.addMonth(1)

Decreasing is also possible by specifying a negative number:

const date = new Date()
d.addMonth(-1)

[xnacly]

LGTM, has jsdoc, has examples, well done