Extraneous names for current state type

A machine's `current` property type ends up containing extra property names. ```ts const machine = createMachine({ a: state(), b: state() }); type Foo = typeof machine['current']; // Expected: // type Foo = 'a' | 'b'; // Actual: // type Foo = 'a' | 'b' | keyof MachineState<string>; // (Expands to): // type Foo = 'a' | 'b' | 'final' | 'transitions' | 'immediate' | 'enter'; ``` I believe this is caused by the change to `createMachine()` type in 4f6fb69. Even though the third type argument to `Machine` in the return value is unchanged: `AllStateKeys<S>`, the type of `S` was changed. Now `S` is a full `MachineStates` object, whose values are objects with keys `final`, `transitions`, `immediate` and `enter`. So when passing a full `MachineState` to `AllStateKeys`, it extracts nested keys too. Thus the result is all the state keys plus the nested `final`, `transitions`, `immediate` and `enter`.