Move nonnull type to public namespace

[veewee]

Currently, the hack in the nonnull type results in a psalm issue when it's being used:

This PR moves the NonNull type out of the internal namespace to overcome this problem.

[coveralls]

Pull Request Test Coverage Report for Build 9437970726

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.04%) to 97.228%

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Files with Coverage Reduction	New Missed Lines	%
src/Psl/DateTime/Internal/high_resolution_time.php	2	80.95%

Totals
Change from base Build 9285294552:	-0.04%
Covered Lines:	5016
Relevant Lines:	5159

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 9442818277

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 97.271%

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Totals
Change from base Build 9442807174:	0.0%
Covered Lines:	5025
Relevant Lines:	5166

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💛 - Coveralls