LibWeb: Start fleshing out the ReadableStream interface

This is so we can just assume it exists in Fetch APIs (while still
skipping functionality that relies on a full implementation, of
course).

Userland/Libraries/LibWeb/Streams/ReadableStream.idl

@@ -0,0 +1,5 @@
+// Dummy definition so we can use ReadableStream as a type in Fetch.
+
+// https://streams.spec.whatwg.org/#readablestream
+interface ReadableStream {
+};