Constexpr fmt::formatter::parse for C++20

[AntoinePrv]

Move some code to make fmt::formatter::parse, which will be required for C++20.

[codecov]

Codecov Report

Attention: Patch coverage is 88.00000% with 9 lines in your changes missing coverage. Please review.

Please upload report for BASE (main@cc7d2d5). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
libmamba/include/mamba/specs/build_number_spec.hpp	75.00%	2 Missing ⚠️
libmamba/include/mamba/specs/version.hpp	75.00%	2 Missing ⚠️
libmamba/src/specs/version_spec.cpp	50.00%	2 Missing ⚠️
...bmamba/include/mamba/specs/chimera_string_spec.hpp	75.00%	1 Missing ⚠️
libmamba/include/mamba/specs/glob_spec.hpp	75.00%	1 Missing ⚠️
libmamba/include/mamba/specs/match_spec.hpp	75.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3942   +/-   ##
=======================================
  Coverage        ?   63.61%           
=======================================
  Files           ?      302           
  Lines           ?    37789           
  Branches        ?     2819           
=======================================
  Hits            ?    24041           
  Misses          ?    13694           
  Partials        ?       54           

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