feature: a new function logic about add months

[background]
In most business scenarios, in order to consider the interests of users, adding one month is generally "month end to month end".
For example, the validity period of some resource packs.

[example]
dayjs('2022-2-28').newAdd(1, 'month').format('YYYY-MM-DD') => 2022-3-31

No file or test is named. Start at the Day.js date-addition API shown in the example and trace the existing month logic. Done means the requested month-end behavior is supported, including the demonstrated 2022-02-28 to 2022-03-31 result, with relevant date arithmetic tests.

Written by the indexing model from the issue text.