LibWeb: Implement '5.3. Body mixin' from the Fetch API :^)

This will be used to share functionality between Request and Response.

Userland/Libraries/LibWeb/Fetch/Body.idl

@@ -0,0 +1,12 @@
+#import <Streams/ReadableStream.idl>
+
+// https://fetch.spec.whatwg.org/#body
+interface mixin Body {
+    readonly attribute ReadableStream? body;
+    readonly attribute boolean bodyUsed;
+    [NewObject] Promise<ArrayBuffer> arrayBuffer();
+    [NewObject] Promise<Blob> blob();
+    [NewObject] Promise<FormData> formData();
+    [NewObject] Promise<any> json();
+    [NewObject] Promise<USVString> text();
+};