Feature: #70002 - Make it possible to register own icons for file extensions

See Issue #70002

Description

The IconRegistry has been extended with a mapping of file extensions.

Impact

It is now possible to register or overwrite the iconIdentifier for a file extension.

$iconRegistry = GeneralUtility::makeInstance(IconRegistry::class);
$iconRegistry->registerFileExtension('log', 'icon-identiifer-for-log-files');