Can add lexical_cast overloads constrained with enable_if

[captainurist]

Also works for overloads constrained with concepts or requirements.

Previously this wasn't working, which is a problem if you want to hook some external serialization framework into CLI11 without painstakingly going through all the types that said external framework supports.

This PR is related to #908.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (e4ee3af) to head (f407d2c).
Report is 13 commits behind head on main.

Additional details and impacted files

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##              main     #1021    +/-   ##
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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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[captainurist]

New Codacy issue is a false positive:

class member 'is_lexical_castable::value' is never used.
static constexpr bool value = decltype(test<T, S>(0))::value;

This class member is, in fact, used.

Fuzz / quickfuzz1 segfault is unrelated to my changes.

[phlptp]

I think I am ok with this. I want to try it out in a few other contexts just to check a few things.

[captainurist]

Any updates here?

[henryiii]

I think this seems fine, waiting on @phlptp to check whatever he was going to check.

[captainurist]

@phlptp any updates here?

[phlptp]

Sorry I am on vacation right now, didn't get a chance to explore before leaving. If you want to merge it I think it is fine, and If I find any issues later I will make some fixes. Otherwise it will be a couple weeks yet.

[captainurist]

OK will ping again in May

[phlptp]

Played with this in a few other applications and tests, no issues came up in compilation or testing so I am going to merge it.