RFC: SchemaExtension

This adds new grammar to the GraphQL SDL:

```graphql
extend schema {
  mutation: MutationType
}
```

This feels like the missing piece of the type system extension framework. In addition to extending types in a type system, the top level schema should be extendable as well.

spec/Appendix B -- Grammar Summary.md

@@ -187,11 +187,19 @@ Directive[Const] : @ Name Arguments[?Const]?
 TypeSystemDefinition :
   - SchemaDefinition
   - TypeDefinition
-  - TypeExtension
   - DirectiveDefinition
+  - TypeSystemExtension
+
+TypeSystemExtension :
+  - SchemaExtension
+  - TypeExtension
 
 SchemaDefinition : schema Directives[Const]? { OperationTypeDefinition+ }
 
+SchemaExtension :
+  - extend schema Directives[Const]? { OperationTypeDefinition+ }
+  - extend schema Directives[Const]
+
 OperationTypeDefinition : OperationType : NamedType
 
 Description : StringValue

spec/Section 3 -- Type System.md

@@ -8,8 +8,8 @@ are valid.
 TypeSystemDefinition :
   - SchemaDefinition
   - TypeDefinition
-  - TypeExtension
   - DirectiveDefinition
+  - TypeSystemExtension
 
 The GraphQL language includes an
 [IDL](https://en.wikipedia.org/wiki/Interface_description_language) used to
@@ -27,6 +27,18 @@ Note: The type system definition language is used throughout the remainder of
 this specification document when illustrating example type systems.
 
 
+## Type System Extensions
+
+TypeSystemExtension :
+  - SchemaExtension
+  - TypeExtension
+
+Type system extensions are used to represent a GraphQL type system which has been
+extended from some original type system. For example, this might be used by a
+local service to represent data a GraphQL client only accesses locally, or by a
+GraphQL service which is itself an extension of another GraphQL service.
+
+
 ## Schema
 
 SchemaDefinition : schema Directives[Const]? { RootOperationTypeDefinition+ }
@@ -139,6 +151,17 @@ type Query {
 }
 ```
 
+### Schema Extension
+
+SchemaExtension :
+  - extend schema Directives[Const]? { OperationTypeDefinition+ }
+  - extend schema Directives[Const]
+
+Schema extensions are used to represent a schema which has been extended from
+some original schema type. For example, this might be used by a GraphQL service
+which adds additional operation types, or additional directives to an existing
+schema.
+
 
 ## Descriptions
 
@@ -261,10 +284,9 @@ TypeExtension :
   - EnumTypeExtension
   - InputObjectTypeExtension
 
-Type extensions are used to represent a GraphQL type system which has been
-extended from some original type system. For example, this might be used by a
-local service to represent data a GraphQL client only accesses locally, or by a
-GraphQL service which is itself an extension of another GraphQL service.
+Type extensions are used to represent a GraphQL type which has been extended
+from some original type. For example, this might be used by a local service to
+represent additional fields a GraphQL client only accesses locally.
 
 
 ## Scalars