Support for <format>

[mborland]

Closes: #341

I am going to leave this one as a draft for now. There are a number of compiler errors since I don't think the support for this is totally complete yet.

[mborland]

Use: https://github.com/boostorg/multiprecision/pull/535/files as reference for implementation

[mborland]

GCC-13 and Clang-17 added full support for <format> so this should be feasible now.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.9%. Comparing base (5ef5f0d) to head (37a0c24).
Report is 35 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #363     +/-   ##
=========================================
+ Coverage     98.7%   98.9%   +0.2%     
=========================================
  Files          223     225      +2     
  Lines        16738   16886    +148     
  Branches      1799    1804      +5     
=========================================
+ Hits         16517   16686    +169     
+ Misses         221     200     -21     

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

... and 1 file with indirect coverage changes

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