LibWeb: Add 'byte-{lower,upper}case' operations from the Infra spec

Usually operations that mirror AOs from the Infra spec are simply part
of the underlying data structures in AK directly, but these don't seem
generally useful enough to add them as ByteBuffer methods.

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -237,6 +237,7 @@ set(SOURCES
     HTML/WorkerLocation.cpp
     HighResolutionTime/Performance.cpp
     ImageDecoding.cpp
+    Infra/ByteSequences.cpp
     IntersectionObserver/IntersectionObserver.cpp
     Layout/BlockContainer.cpp
     Layout/BlockFormattingContext.cpp