Support optional keys in record types

[soutaro]

The semantics of optional keys in record in Steep is the hash may not have any value associated with the optional key.

So,

1. Assigning a hash without the optional key is allowed -- { id: 123 } #: { id: Integer, ?name: String }
2. Having incompatible type with the optional key is prohibited (but, it causes unsoundness...)
3. The #[] operator of the record type returns an optional type, #fetch returns non-optional type (if the value type is not an optional)

[ParadoxV5]

• Record type with optional keys ruby/rbs#504