[graphql] Predicates can be now used as type guards (#31701)

* feat(graphql): predicates as type guards

Predicates in `graphql/language/predicates` are used in Flow as type
guards.
Now they can be used in TS too:

* isDefinitionNode
* isExecutableDefinitionNode
* isSelectionNode
* isValueNode
* isTypeNode
* isTypeSystemDefinitionNode
* isTypeDefinitionNode
* isTypeSystemExtensionNode
* isTypeExtensionNode

* chore(graphql): add langpavel to "Definitions by"

types/graphql/index.d.ts

@@ -16,6 +16,7 @@
 //                 Brad Zacher <https://github.com/bradzacher>
 //                 Curtis Layne <https://github.com/clayne11>
 //                 Jonathan Cardoso <https://github.com/JCMais>
+//                 Pavel Lang <https://github.com/langpavel>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 // TypeScript Version: 2.6
 

types/graphql/language/predicates.d.ts

@@ -1,19 +1,30 @@
-import { ASTNode } from "./ast";
+import {
+    ASTNode,
+    DefinitionNode,
+    ExecutableDefinitionNode,
+    SelectionNode,
+    ValueNode,
+    TypeNode,
+    TypeSystemDefinitionNode,
+    TypeDefinitionNode,
+    TypeSystemExtensionNode,
+    TypeExtensionNode,
+} from "./ast";
 
-export function isDefinitionNode(node: ASTNode): boolean;
+export function isDefinitionNode(node: ASTNode): node is DefinitionNode;
 
-export function isExecutableDefinitionNode(node: ASTNode): boolean;
+export function isExecutableDefinitionNode(node: ASTNode): node is ExecutableDefinitionNode;
 
-export function isSelectionNode(node: ASTNode): boolean;
+export function isSelectionNode(node: ASTNode): node is SelectionNode;
 
-export function isValueNode(node: ASTNode): boolean;
+export function isValueNode(node: ASTNode): node is ValueNode;
 
-export function isTypeNode(node: ASTNode): boolean;
+export function isTypeNode(node: ASTNode): node is TypeNode;
 
-export function isTypeSystemDefinitionNode(node: ASTNode): boolean;
+export function isTypeSystemDefinitionNode(node: ASTNode): node is TypeSystemDefinitionNode;
 
-export function isTypeDefinitionNode(node: ASTNode): boolean;
+export function isTypeDefinitionNode(node: ASTNode): node is TypeDefinitionNode;
 
-export function isTypeSystemExtensionNode(node: ASTNode): boolean;
+export function isTypeSystemExtensionNode(node: ASTNode): node is TypeSystemExtensionNode;
 
-export function isTypeExtensionNode(node: ASTNode): boolean;
+export function isTypeExtensionNode(node: ASTNode): node is TypeExtensionNode;