Add IWYU pragmas

[gostefan]

Added include-what-you-use pragmas to:

• let IWYU point users to the main include file CLI/CLI.hpp
• tell IWYU that CLI/CLI.hpp is the main exporting header.

This should fix #816

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (e4ee3af) to head (3e7e3e2).
Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##              main     #1008    +/-   ##
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  Coverage   100.00%   100.00%            
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  Files           17        17            
  Lines         4546      4557    +11     
  Branches         0       971   +971     
==========================================
+ Hits          4546      4557    +11     

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[phlptp]

This looks OK to me. I am going to give @henryiii a few days to comment, then will merge.

[henryiii]

Not very familiar with IWYU pragmas, but seems fine.

[henryiii]

Oh, it is valid though to include individual headers (it's in the readme). It sounds like this will just point users to always including the conglomerate header? Shouldn't these (Config and formatter) have // IWYU pragma: keep instead?

[gostefan]

I looked through the Readme and didn't find any references to individually including headers except for the utilities (which I promptly forgot to correctly handle in my changes).

Is it ok to include the "regular" headers (e.g. Argv.hpp) individually?
I'll adapt the PR accordingly.