ci: set up coverage on GHA

[henryiii]

This has been missing since we dropped Travis CI.

[codecov]

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@@           Coverage Diff           @@
##             main     #747   +/-   ##
=======================================
  Coverage        ?   99.46%           
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  Files           ?       12           
  Lines           ?     3958           
  Branches        ?        0           
=======================================
  Hits            ?     3937           
  Misses          ?       21           
  Partials        ?        0           

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[henryiii]

This is a little worrisome:

D:\a\1\s\tests\HelpersTest.cpp(1231,82): error C2668: 'CLI::detail::lexical_conversion': ambiguous call to overloaded function [D:\a\1\s\build\tests\HelpersTest.vcxproj]
D:\a\1\s\include\CLI\TypeTools.hpp(1438,6): message : could be 'bool CLI::detail::lexical_conversion<std::array<double,4>,std::array<double,4>,(CLI::detail::enabler)0>(const std::vector<std::string,std::allocator<std::string>> &,AssignTo &)' [D:\a\1\s\build\tests\HelpersTest.vcxproj]
          with
          [
              AssignTo=std::array<double,4>
          ]
D:\a\1\s\include\CLI\TypeTools.hpp(1200,6): message : or       'bool CLI::detail::lexical_conversion<std::array<double,4>,std::array<double,4>,(CLI::detail::enabler)0>(const std::vector<std::string,std::allocator<std::string>> &,AssignTo &)' [D:\a\1\s\build\tests\HelpersTest.vcxproj]
          with
          [
              AssignTo=std::array<double,4>
          ]
D:\a\1\s\tests\HelpersTest.cpp(1231,82): message : while trying to match the argument list '(CLI::results_t, std::array<double,4>)' [D:\a\1\s\build\tests\HelpersTest.vcxproj]
D:\a\1\s\tests\HelpersTest.cpp(1239,77): error C2668: 'CLI::detail::lexical_conversion': ambiguous call to overloaded function [D:\a\1\s\build\tests\HelpersTest.vcxproj]
D:\a\1\s\include\CLI\TypeTools.hpp(1438,6): message : could be 'bool CLI::detail::lexical_conversion<std::array<double,4>,std::array<double,4>,(CLI::detail::enabler)0>(const std::vector<std::string,std::allocator<std::string>> &,AssignTo &)' [D:\a\1\s\build\tests\HelpersTest.vcxproj]
          with
          [
              AssignTo=std::array<double,4>
          ]
D:\a\1\s\include\CLI\TypeTools.hpp(1200,6): message : or       'bool CLI::detail::lexical_conversion<std::array<double,4>,std::array<double,4>,(CLI::detail::enabler)0>(const std::vector<std::string,std::allocator<std::string>> &,AssignTo &)' [D:\a\1\s\build\tests\HelpersTest.vcxproj]
          with
          [
              AssignTo=std::array<double,4>
          ]
D:\a\1\s\tests\HelpersTest.cpp(1239,77): message : while trying to match the argument list '(CLI::results_t, std::array<double,4>)' [D:\a\1\s\build\tests\HelpersTest.vcxproj]

This is the single header file on the top, I think it's the same error. Happens on C++latest/C++20 + windows-2022. I'll work around it for this PR by going back to windows-2019, but needs to be looked at.