Use interface mixins instead of [NoInterfaceObject]

WebIDL recently introduced dedicated syntax for mixins[1]. So, we can
replace `[NoInterfaceObject]` and `implements` with `interface mixin` and
`includes`.

This following interface is impacted by this change:
  - Body

This fixes whatwg#624 issue.

[1] whatwg/webidl@45e8173

fetch.bs

@@ -4660,9 +4660,8 @@ typedef (Blob or BufferSource or FormData or URLSearchParams or ReadableStream o
 </ol>
 
 
-<pre class=idl>[NoInterfaceObject,
- Exposed=(Window,Worker)]
-interface Body {
+<pre class=idl>
+interface mixin Body {
   readonly attribute ReadableStream? body;
   readonly attribute boolean bodyUsed;
   [NewObject] Promise&lt;ArrayBuffer> arrayBuffer();
@@ -4856,7 +4855,8 @@ interface Request {
 
   [NewObject] Request clone();
 };
-Request implements Body;
+Request includes Body;
+
 dictionary RequestInit {
   ByteString method;
   HeadersInit headers;
@@ -5418,7 +5418,7 @@ interface Response {
 
   [NewObject] Response clone();
 };
-Response implements Body;
+Response includes Body;
 
 dictionary ResponseInit {
   unsigned short status = 200;
@@ -6124,6 +6124,7 @@ Jeff Carpenter,
 Jeff Hodges,
 Jeffrey Yasskin,
 Jesse M. Heines,
+Jinho Bang,
 Jochen Eisinger,
 Jonas Sicking,
 Jonathan Kingston,