Add JsonUtils::isValidJson #56

Job	Run time
Check Coding Standards with php-cs-fixer (locked, 8.1, ubuntu-latest)	34s
Check Coding Standards with PHP_CodeSniffer (locked, 8.1, ubuntu-latest)	18s
Static Analysis by PHPStan (locked, 8.1, ubuntu-latest)	12s
PHPUnit tests (lowest, 8.1, ubuntu-latest)	11s
PHPUnit tests (lowest, 8.2, ubuntu-latest)	17s
PHPUnit tests (lowest, 8.3, ubuntu-latest)	12s
PHPUnit tests (highest, 8.1, ubuntu-latest)	20s
PHPUnit tests (highest, 8.2, ubuntu-latest)	11s
PHPUnit tests (highest, 8.3, ubuntu-latest)	18s
PHPUnit tests (locked, 8.1, ubuntu-latest)	10s
PHPUnit tests (locked, 8.2, ubuntu-latest)	11s
PHPUnit tests (locked, 8.3, ubuntu-latest)	11s
	3m 5s

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