(fix): don't return url for cpt's with publicly_queryable false

[rivanuff]

See #54

[github-actions]

Coverage report for commit: 49975ce
File: coverage.xml

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 *Legend:* █ = Current Distribution 

Summary - Lines: 13.53% | Methods: 9.09%

Files	Lines	Methods	Branches
src/Attributes
Meta.php	-	-	100.00%
MetaPrefix.php	-	-	100.00%
TaxonomyPrefix.php	-	-	100.00%
Terms.php	-	-	100.00%
src/Contracts
PostDataInterface.php	100.00%	100.00%	100.00%
src/Enums
PostStatus.php	-	-	100.00%
src
ImageData.php	-	-	100.00%
PostData.php	18.26%	8.70%	100.00%
TermData.php	-	-	100.00%
UserData.php	-	-	100.00%
src/Mappers
PostPrefixMapper.php	-	-	100.00%
UserPrefixMapper.php	-	-	100.00%
src/Normalizers
WPPostNormalizer.php	-	-	100.00%
src/Providers
DataServiceProvider.php	100.00%	100.00%	100.00%

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[SimonvanWijhe]

The url() method is currently just a convenience helper, allowing you to use $post->url() instead of get_permalink($post->id()).

From my perspective, the use of is_post_type_viewable() here seems like an attempt to work around a limitation in get_permalink() rather than something that belongs in the url() method itself.