fix(formats): avoid using internal unit in add_unit()

[nijel]

Proposed changes

This breaks in multivalue storage and is using internals where not necessary.

Fixes WEBLATE-CH

Checklist

• Lint and unit tests pass locally with my changes.
• I have added tests that prove my fix is effective or that my feature works.
• I have added documentation to describe my feature.
• I have squashed my commits into logic units.
• I have described the changes in the commit messages.

Other information

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 4 lines in your changes missing coverage. Please review.

Project coverage is 90.57%. Comparing base (6aee586) to head (77b4f94).
Report is 2135 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #11847      +/-   ##
==========================================
- Coverage   90.82%   90.57%   -0.25%     
==========================================
  Files         554      576      +22     
  Lines       57306    58707    +1401     
  Branches     9122     9386     +264     
==========================================
+ Hits        52046    53173    +1127     
- Misses       3640     3835     +195     
- Partials     1620     1699      +79     

Files	Coverage Δ
weblate/formats/base.py	89.86% <100.00%> (+0.08%)	⬆️
weblate/formats/tests/test_formats.py	99.76% <100.00%> (+<0.01%)	⬆️
weblate/formats/ttkit.py	90.84% <100.00%> (-0.57%)	⬇️
weblate/formats/txt.py	79.87% <100.00%> (-0.52%)	⬇️
weblate/trans/models/translation.py	86.71% <100.00%> (-0.50%)	⬇️
weblate/formats/convert.py	87.64% <50.00%> (-1.22%)	⬇️
weblate/formats/multi.py	83.62% <40.00%> (-3.88%)	⬇️

... and 316 files with indirect coverage changes