Add an "end of period" date modifier

I find there are a number of cases where one may want to set a `due` date *at the end* of a given period, e.g. to add a task due:

* today, end of day
* tomorrow, end of day
* March, end of month
* 2024, end of year

There are several `eo-` shortcuts covering some of these cases (https://github.com/GothenburgBitFactory/libshared/blob/master/src/Datetime.cpp#L551), but they mostly help setting a date at the end of a given *day*. What I find is missing is a general "end of period" modifier, which completes the unspecified parts of a date with the latest possible value, instead of the earliest one.

Some examples with clarify. Let's assume the "end of period" modifier is `]`. Then:

* `due:today]` means today at 23:59:59
* `due:tomorrow]` means tomorrow at 23:59:59
* `due:march]` means `2022-03-31 23:59:59`
* `due:2024]` means `2024-12-31 23:59-59`

Something similar was already mentioned here: https://github.com/GothenburgBitFactory/taskwarrior/issues/179#issuecomment-365012602, but that was about an "end of day" modifier, not an "end of period".