fix(Answer placeholders OOP Sealed Classes #1): Added answer placeholders

[sofiiako]

Placeholders corresponding to the things mentioned in the description were missing, there was a giant one covering the whole task with no starter code shown to the student.

[svtk]

The preview file should compile with answer placeholders, otherwise, the whole project doesn't compile and the students receive an error "Compilation failed" on any check. Fixed it here https://github.com/svtk/AtomicKotlinCourse/commit/36319bf5d45b2c264d3889754307fb7bb7d99fd4