Add Type parsing

[Morriar]

This PR adds parsing for the Type builder introduced in #331.

This allows the user to parse types from a string and get a RBI::Type from them:

type = Type.parse_string("T.nilable(Foo)")
type.class # => Type::Nilable
type.type # => Type::Simple("Foo")

Note that we do not migrate the RBI::Parser to parse types at this point, the general parser still uses Strings to represent types in signatures and T::Structs.

cc. @paracycle

[paracycle]

I know you had concerns about the parser potentially raising when parsing non-sensical input instead of parsing it as is and allowing another system to show errors.

For that reason, I wonder if we should use the constructors directly here, instead of using the helper methods.

[Morriar]

I'm still conflicted on the choice with made regarding the builder API.

Calling new directly is a good escape hatch but by making it private we made it impossible to use for external users. Maybe the right thing to do would be to keep the new public?

In the meantime you're right, I changed all the calls from the parser to use new directly rather than the builder interface.

[paracycle]

Agreed, but it is easier to make the new public later than to make it private later. So, I think I like how we are exposing a more minimal API for the time being.