fix(format/writer): strings longer than the buffer chunk size are written correctly

[Viatorus]

Fixes #68

[codecov-commenter]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (423edc9) 99.40% compared to head (40fffb4) 99.40%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #71   +/-   ##
=======================================
  Coverage   99.40%   99.40%           
=======================================
  Files          56       56           
  Lines        5866     5869    +3     
=======================================
+ Hits         5831     5834    +3     
  Misses         35       35           

Files	Coverage Δ
include/emio/writer.hpp	100.00% <100.00%> (ø)
test/unit_test/test_writer.cpp	100.00% <100.00%> (ø)

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