Add cohort preserving to_chars with scientific format

[mborland]

Closes: #1150

[codecov]

Codecov Report

❌ Patch coverage is 96.72131% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.4%. Comparing base (8102c85) to head (bbf92c5).
⚠️ Report is 39 commits behind head on develop.

Files with missing lines	Patch %	Lines
include/boost/decimal/charconv.hpp	93.6%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1157     +/-   ##
=========================================
- Coverage     99.4%   99.4%   -0.0%     
=========================================
  Files          252     253      +1     
  Lines        17208   17269     +61     
  Branches      1870    1880     +10     
=========================================
+ Hits         17091   17149     +58     
- Misses         117     120      +3     

Files with missing lines	Coverage Δ
include/boost/decimal/decimal128_t.hpp	98.7% <ø> (ø)
include/boost/decimal/decimal32_t.hpp	96.8% <ø> (ø)
include/boost/decimal/decimal64_t.hpp	96.6% <ø> (ø)
test/test_to_chars_quantum.cpp	100.0% <100.0%> (ø)
include/boost/decimal/charconv.hpp	97.6% <93.6%> (-0.2%)	⬇️

... and 1 file with indirect coverage changes

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