Fix inability to express a positive offset of less than an hour

[Baptouuuu]

Problem

With the current API Offset::of(0, 30) is interpreted as -00:30.

Solution

Use explicit ::plus and ::minus named constructors.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.91%. Comparing base (f260ba7) to head (12382d5).

Additional details and impacted files

@@            Coverage Diff             @@
##             develop      #13   +/-   ##
==========================================
  Coverage      99.91%   99.91%           
- Complexity       694      695    +1     
==========================================
  Files             54       54           
  Lines           2246     2248    +2     
==========================================
+ Hits            2244     2246    +2     
  Misses             2        2           

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