[TypeScript 3.0]: New tuple type support

[DanielRosenwasser]

TypeScript 3.0 eased one restriction and added support for two new features in tuple types:

1. Tuples types are allowed to be empty (already works in Babel), so [] is a valid type.
2. Tuples elements are allowed to be optional (so each element in a tuple can have a trailing ?).
3. Tuples are allowed to have a trailing rest element (the last element can have a leading ...).

The following example uses all of these constructs:

declare function foo(ignored: [], ...args: [number?, ...number[]]);

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