LibWeb: Add the WritableStream interface

Userland/Libraries/LibWeb/Streams/WritableStream.idl

@@ -0,0 +1,12 @@
+[Exposed=*, Transferable]
+interface WritableStream {
+    // FIXME: optional QueuingStrategy strategy = {}
+    constructor(optional object underlyingSink);
+
+    readonly attribute boolean locked;
+
+    // FIXME:
+    // Promise<undefined> abort(optional any reason);
+    // Promise<undefined> close();
+    // WritableStreamDefaultWriter getWriter();
+};