Disallow alternative control structure syntax

[piqusy]

Summary

• Adds Eightshift.ControlStructures.DisallowAlternativeSyntax sniff that errors on endif, endforeach, endfor, endwhile, and endswitch — enforcing curly braces for all control structures
• Fixes phpunit.xml.dist to point to AllSniffs.php as the test suite entry point (fixes pre-existing Undefined global variable $PHP_CODESNIFFER_STANDARD_DIRS error affecting all tests)
• Includes unit test fixture and documentation

Test plan

• ./vendor/bin/phpunit — all 4 tests pass
• ./vendor/bin/phpcs --standard=Eightshift on a file using endif/endforeach — errors reported
• ./vendor/bin/phpcs --standard=Eightshift on a file using curly braces only — clean