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.

Tests/LibWeb/TestHTMLTokenizer.cpp

@@ -81,7 +81,7 @@ static u32 hash_tokens(Vector<Token> const& tokens)
 {
     StringBuilder builder;
     for (auto& token : tokens)
-        builder.append(token.to_string());
+        builder.append(token.to_deprecated_string());
     return (u32)builder.string_view().hash();
 }