Add sto* family of std::string conversion functions

[mborland]

Closes: #1270

[cppalliance-bot]

An automated preview of the documentation is available at https://1272.decimal.prtest3.cppalliance.org/libs/decimal/doc/html/index.html

If more commits are pushed to the pull request, the docs will rebuild at the same URL.

2025-12-05 13:57:50 UTC

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.9%. Comparing base (2bc8e5f) to head (0d0bd12).
⚠️ Report is 7 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1272     +/-   ##
=========================================
+ Coverage     98.9%   98.9%   +0.1%     
=========================================
  Files          266     267      +1     
  Lines        17698   17755     +57     
  Branches      1970    1973      +3     
=========================================
+ Hits         17492   17550     +58     
+ Misses         206     205      -1     

Files with missing lines	Coverage Δ
include/boost/decimal/string.hpp	100.0% <100.0%> (ø)
test/test_from_string.cpp	100.0% <100.0%> (ø)

... and 2 files with indirect coverage changes

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