Add StateMachineBuilder trait

[yuk1ty]

Motivation

Now we don't have any way to expand our own StateMachineBuilder because the builder is struct. There is no correct way to expand its builder according to the user's own implementation when the users create their own StateMachine and want to expand the builder to be able to assemble their StateMachine.

Todo

-[x] Add StateMachineBuilder trait.
- due to this issue, I can no longer use mut self in the functions, so I'll change it to self.
-[x] Rename old StateMachineBuilder to BasicMachineBuilder (because the builder is to create BasicStateMachine).
-[x] Update docs.