Avoid parse ambiguity on type extensions

Partial fix to #564, adds lookahead constraints to type system extensions to remove ambiguity or inefficiency from the grammar.

The GraphQL grammar should be parsed in linear type with at most one lookahead. Since extensions have an optional `{ }` body, finding the token `{` should always attempt to parse the relevant portion of the type extension rather than completing the type extension and attempting to parse the query shorthand SelectionSet.

Author: leebyron

spec/Section 3 -- Type System.md

@@ -154,12 +154,16 @@ type Query {
 
 SchemaExtension :
   - extend schema Directives[Const]? { RootOperationTypeDefinition+ }
-  - extend schema Directives[Const]
+  - extend schema Directives[Const] [lookahead != SelectionSet]
 
 Schema extensions are used to represent a schema which has been extended from
 an original schema. For example, this might be used by a GraphQL service which
 adds additional operation types, or additional directives to an existing schema.
 
+Note: Extensions without additional operation types must not be followed by a
+{SelectionSet} query shorthand to avoid parsing ambiguity. This same limitation
+occurs on the type extensions below.
+
 **Schema Validation**
 
 Schema extensions have the potential to be invalid if incorrectly defined.
@@ -902,8 +906,8 @@ type ExampleType {
 
 ObjectTypeExtension :
   - extend type Name ImplementsInterfaces? Directives[Const]? FieldsDefinition
-  - extend type Name ImplementsInterfaces? Directives[Const]
-  - extend type Name ImplementsInterfaces
+  - extend type Name ImplementsInterfaces? Directives[Const] [lookahead != SelectionSet]
+  - extend type Name ImplementsInterfaces [lookahead != SelectionSet]
 
 Object type extensions are used to represent a type which has been extended from
 some original type. For example, this might be used to represent local data, or
@@ -1071,7 +1075,7 @@ Interface types have the potential to be invalid if incorrectly defined.
 
 InterfaceTypeExtension :
   - extend interface Name Directives[Const]? FieldsDefinition
-  - extend interface Name Directives[Const]
+  - extend interface Name Directives[Const] [lookahead != SelectionSet]
 
 Interface type extensions are used to represent an interface which has been
 extended from some original interface. For example, this might be used to
@@ -1294,7 +1298,7 @@ Enum types have the potential to be invalid if incorrectly defined.
 
 EnumTypeExtension :
   - extend enum Name Directives[Const]? EnumValuesDefinition
-  - extend enum Name Directives[Const]
+  - extend enum Name Directives[Const] [lookahead != SelectionSet]
 
 Enum type extensions are used to represent an enum type which has been
 extended from some original enum type. For example, this might be used to
@@ -1424,7 +1428,7 @@ Literal Value            | Variables               | Coerced Value
 
 InputObjectTypeExtension :
   - extend input Name Directives[Const]? InputFieldsDefinition
-  - extend input Name Directives[Const]
+  - extend input Name Directives[Const] [lookahead != SelectionSet]
 
 Input object type extensions are used to represent an input object type which
 has been extended from some original input object type. For example, this might