LibWeb: Port CharacterData from DeprecatedString to String

The existing implementation has some pre-existing issues where it is
incorrectly assumes that byte offsets are given through the IDL instead
of UTF-16 code units. While making these changes, leave some FIXMEs for
that.

Userland/Libraries/LibWeb/DOM/Position.cpp

@@ -67,7 +67,7 @@ bool Position::offset_is_at_end_of_node() const
 
     auto& node = verify_cast<DOM::Text>(*m_node);
     auto text = node.data();
-    return m_offset == text.length();
+    return m_offset == text.bytes_as_string_view().length();
 }
 
 }