String is a reserved keyword in PHP 7

Problem/Motivation

PHP_CodeSniffer finds:

FILE: src/Plugin/GraphQL/DataProducer/String/Uppercase.php
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FOUND 1 ERROR AFFECTING 1 LINE
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3 | ERROR | 'string' is a reserved keyword as of PHP version 7.0 and should
|       | not be used to name a class, interface or trait or as part of a
|       | namespace (T_NAMESPACE)
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Steps to reproduce

Proposed resolution

Move this Uppercase class to another namespace instead of String.

Remaining tasks

User interface changes

API changes

Data model changes