LibWeb: Properly convert UnderlyingSource's autoAllocateChunkSize to u64

The JS::Value being passed through is not a bigint, and needs to be
converted using ConvertToInt, as per:

https://webidl.spec.whatwg.org/#es-unsigned-long-long

Furthermore, the IDL definition also specifies that this is associated
with the [EnforceRange] extended attribute.

This makes it actually possible to pass through an autoAllocateChunkSize
to the ReadableStream constructor without it throwing a TypeError.

Tests/LibWeb/Text/input/Streams/ReadableStream-autoAllocateChunkSize-constructor.html

@@ -0,0 +1,10 @@
+<script src="../include.js"></script>
+<script>
+    test(() => {
+        let stream = new ReadableStream({
+            type: "bytes",
+            autoAllocateChunkSize: 64
+        });
+        println(`PASS. Made: ${stream.constructor.name}`);
+    });
+</script>