Everywhere: Rename to_{string => deprecated_string}() where applicable

This will make it easier to support both string types at the same time
while we convert code, and tracking down remaining uses.

One big exception is Value::to_string() in LibJS, where the name is
dictated by the ToString AO.

Userland/Libraries/LibWeb/HTML/Parser/HTMLTokenizer.cpp

@@ -2887,7 +2887,7 @@ void HTMLTokenizer::restore_to(Utf8CodePointIterator const& new_iterator)
 
 DeprecatedString HTMLTokenizer::consume_current_builder()
 {
-    auto string = m_current_builder.to_string();
+    auto string = m_current_builder.to_deprecated_string();
     m_current_builder.clear();
     return string;
 }