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/DevTools/HackStudio/FindInFilesWidget.cpp

@@ -105,7 +105,7 @@ static RefPtr<SearchResultsModel> find_in_files(StringView text)
             builder.append(file.document().text_in_range(range));
             builder.append(0x02);
             builder.append(right_part);
-            matches.append({ file.name(), range, builder.to_string() });
+            matches.append({ file.name(), range, builder.to_deprecated_string() });
         }
     });