Add utility methods for casting resources to their particular types

[vzhukovs]

Add to org.eclipse.che.ide.api.resources.Resource interface utility methods to cast current resource to its particular type.

Proposed method names:

• org.eclipse.che.ide.api.resources.Resource#asFile
• org.eclipse.che.ide.api.resources.Resource#asFolder
• org.eclipse.che.ide.api.resources.Resource#asProject

It's syntax sugar to better code readability.

Example of usage:

    public void doSmth() {

        Resource resource = ...;
        
        if (resource.isFolder()) {
            final Folder folder = resource.asFolder();
            
            folder.newFile("name", "content");
        }
        
    }

If resource type is not corresponding caller method, method should throw java.lang.IllegalStateException