Kernel: Add File::is_regular_file()

This makes it easy and expressive to check if a File is a regular file.

Kernel/FileSystem/InodeFile.h

@@ -49,6 +49,8 @@ public:
     virtual bool is_inode() const override { return true; }
 
 private:
+    virtual bool is_regular_file() const override;
+
     explicit InodeFile(NonnullLockRefPtr<Inode>&&);
     NonnullLockRefPtr<Inode> m_inode;
 };