LibWeb/FileAPI: Update Blob.slice for spec changes

Author: https://github.com/jamierocks
Commit: 9650a5ff33
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/1164
Reviewed-by: https://github.com/tcl3 ✅

Userland/Libraries/LibWeb/FileAPI/Blob.h

@@ -67,6 +67,8 @@ protected:
 
     virtual void initialize(JS::Realm&) override;
 
+    WebIDL::ExceptionOr<JS::NonnullGCPtr<Blob>> slice_blob(Optional<i64> start = {}, Optional<i64> end = {}, Optional<String> const& content_type = {});
+
     ByteBuffer m_byte_buffer {};
     String m_type {};