Fix callable type mapping to be Closure mapping

[oprypkhantc]

Quite accidentally I discovered a bug with my implementation: if a value that is also a callable is returned from a field, it will get wrapped in a Deferred wrapper and be called as a callable, instead of simply being returned as a value.

E.g.:

function test() {
    throw new RuntimeException('This is called!');
}

#[Type]
class Test {
    #[Field]
    public string $title = 'test';
}

class TestController {
    #[Query]
    public function test(): Test {
        return new Test();
    }
}

If you then run the test { title } query, instead of simply returning the {"title": "test"}, it would call the test function - because is_callable('test') returns true.

Potential fixes

This could be solved two ways:

1. remove support for callable, only leave support for actual Closure
2. keep support for callable, but make it so it wraps values in Deferred based on whether the type was specified as callable, not if the value itself is callable

This PR implements option #1. Option #2 would require some architectural changes to how GraphQLite works, way outside the scope of this feature :)

[oprypkhantc]

@oojacoboo It's likely that the issue you described in #739 was due to this. It would be hard to catch otherwise, because it's caused by specific values returned from fields

[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.73684% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.83%. Comparing base (53f9d49) to head (d0d5c9a).
⚠️ Report is 118 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Mappers/Root/ClosureTypeMapper.php	92.85%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #753      +/-   ##
============================================
- Coverage     95.72%   94.83%   -0.90%     
- Complexity     1773     1845      +72     
============================================
  Files           154      175      +21     
  Lines          4586     5013     +427     
============================================
+ Hits           4390     4754     +364     
- Misses          196      259      +63     

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[oojacoboo]

This is a good safe option - thanks! And yes, this was the issue with #739