LibWeb: Implement ReadableStream's constructor

Author: https://github.com/mattco98
Commit: d8710aa604
Pull-request: https://github.com/SerenityOS/serenity/pull/18109
Reviewed-by: https://github.com/linusg ✅

Userland/Libraries/LibWeb/Streams/ReadableStream.idl

@@ -3,4 +3,6 @@
 // https://streams.spec.whatwg.org/#readablestream
 [Exposed=*, Transferable]
 interface ReadableStream {
+    // FIXME: optional QueuingStrategy strategy = {}
+    constructor(optional object underlyingSource);
 };