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/Infra/ByteSequences.cpp

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2022, Linus Groh <linusg@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/CharacterTypes.h>
+#include <LibWeb/Infra/ByteSequences.h>
+
+namespace Web::Infra {
+
+// https://infra.spec.whatwg.org/#byte-lowercase
+void byte_lowercase(ByteBuffer& bytes)
+{
+    // To byte-lowercase a byte sequence, increase each byte it contains, in the range 0x41 (A) to 0x5A (Z), inclusive, by 0x20.
+    for (size_t i = 0; i < bytes.size(); ++i)
+        bytes[i] = to_ascii_lowercase(bytes[i]);
+}
+
+// https://infra.spec.whatwg.org/#byte-uppercase
+void byte_uppercase(ByteBuffer& bytes)
+{
+    // To byte-uppercase a byte sequence, subtract each byte it contains, in the range 0x61 (a) to 0x7A (z), inclusive, by 0x20.
+    for (size_t i = 0; i < bytes.size(); ++i)
+        bytes[i] = to_ascii_uppercase(bytes[i]);
+}
+
+}