Adjust check scripts to be case insensitive

[PKief]

The check scripts do not recognize conflicts where the difference between the associations is only based on the capitalization. For example there're two configurations "hpp" and "HPP" for two different icons. Then the one that comes at last in the configuration will be the icon that is shown in the editor (e.g. "HPP"). The issue is that VS Code is not case sensitive regarding the icon file extensions, i.e. "hpp" is equals to "HPP". The check script should throw an error if the file extension appears twice with a different capitalization.