Support optional chaining

[Conduitry]

In order to support optional chaining in Svelte, we need to support it here and in periscopic first.

The PR estree/estree#204 has not been merged yet but the spec apparently is final. Roughly, there's a new ChainExpression type that only has an expression key, and it's this expression that contains the chain of member and call expressions with a new optional boolean on each one.

Before this can be done properly, we might need to wait for @types/estree to be released.