Add signature for formData types (Fetch API)

Author: alfaslash

baselines/dom.generated.d.ts

@@ -13474,6 +13474,7 @@ interface Body {
     blob(): Promise<Blob>;
     json(): Promise<any>;
     text(): Promise<string>;
+    formData(): Promise<FormData>;
 }
 
 interface CanvasPathMethods {

baselines/webworker.generated.d.ts

@@ -1134,6 +1134,7 @@ interface Body {
     blob(): Promise<Blob>;
     json(): Promise<any>;
     text(): Promise<string>;
+    formData(): Promise<FormData>;
 }
 
 interface GlobalFetch {

inputfiles/addedTypes.json

@@ -1437,5 +1437,11 @@
             "name": "reason?",
             "type": "any"
         }]
+    },
+    {
+        "kind": "method",
+        "interface": "Body",
+        "name": "formData",
+        "signatures": ["formData(): Promise<FormData>"]
     }
 ]