Fixes Parser & Printer for SDL Functionality

[NeedleInAJayStack]

This makes the following minor fixes to the Printer and Parser to support SDL functionality:

1. Schema extension parser correctly captures operations
2. Description printing matches reference implementation
3. Parser correctly errors on empty extend blocks
4. Parser correctly errors when disallowed descriptions are provided.

[paulofaria]

LGTM! Nice work. Does this mean we can now print SDL from schemas? I remember I was working on something like this on a very old branch. The one that also had some work related to client and server directives. One of the goals of that branch was also to polish Graphiti's APIs to make them even closer to the SDL. One thing that I really liked was allowing strings on top of type definitions and treating them as block comment descriptions of the types. I should try and cherry pick some of that work and open a PR. There were some interesting stuff there.

[NeedleInAJayStack]

Does this mean we can now print SDL from schemas?

Yes! Well kinda. The last step is just to make it public:

GraphQL/Sources/GraphQL/Language/Printer.swift

Line 5 in 75cfce2

func print(ast: Node) -> String {

I should try and cherry pick some of that work and open a PR

Sounds very cool! I'd love to review it!