Ensure PHPCS handles CLI arguments case-insensitively across operating systems

[rodrigoprimo]

PHPCS CLI arguments should be handled case-insensitively (see squizlabs/PHP_CodeSniffer#1656 (comment) and #771 (comment)). But on #771, it was discovered that PHPCS was handling the --generator argument case-insensitively on Windows, but not on Linux (and probably not on Mac as well).

In this issue, we should check the other PHPCS CLI arguments to ensure they are handled case-insensitively across different operating systems. The --generator argument will be fixed in #771.

There is at least one other argument where this is not happening that needs to be fixed. On Linux, bin/phpcs --filter=GitModified . works, but bin/phpcs --filter=gitmodified . causes a fatal error:

$ bin/phpcs --filter=gitmodified .       
PHP Fatal error:  Uncaught Error: Class "\PHP_CodeSniffer\Filters\gitmodified" not found in src/Files/FileList.php:90
Stack trace:
#0 src/Runner.php(402): PHP_CodeSniffer\Files\FileList->__construct()
#1 src/Runner.php(120): PHP_CodeSniffer\Runner->run()
#2 bin/phpcs(14): PHP_CodeSniffer\Runner->runPHPCS()
#3 {main}
  thrown in src/Files/FileList.php on line 90