allow negative duration via internal API

[kevinpapst]

Description

This is NOT about entering negative duration via the UI, but ONLY via the internal API.

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist

• I verified that my code applies to the guidelines (composer code-check)
• I agree that this code is used in Kimai and will be published under the MIT license

[codecov]

Codecov Report

Merging #3573 (78182de) into master (1959d4d) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #3573      +/-   ##
============================================
- Coverage     91.74%   91.74%   -0.01%     
+ Complexity     7893     7891       -2     
============================================
  Files           744      744              
  Lines         23806    23805       -1     
============================================
- Hits          21842    21841       -1     
  Misses         1964     1964              

Impacted Files	Coverage Δ
src/Entity/Timesheet.php	98.93% <ø> (ø)
src/Utils/LocaleHelper.php	100.00% <ø> (ø)
src/Validator/Constraints/Duration.php	100.00% <ø> (ø)
src/Utils/Duration.php	100.00% <100.00%> (ø)
.../Validator/Constraints/TimesheetBasicValidator.php	95.19% <100.00%> (ø)