Another step towards breaking this out into packages: move the
`CharacterStream` type into `types/Types.js` as an interface. Requires
switching to property syntax to avoid some Flow complaints about
covariance vs invariance.
With this change, `Types` has no dependencies inside of the language
service, which means we can extract it.
Bonus along the way: shortened some simple methods into one liners,
seeing as we have arrow functions and single expressions, so we may as
well just return the expression directly.