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/LibDiff/Format.cpp

@@ -18,6 +18,6 @@ DeprecatedString generate_only_additions(DeprecatedString const& text)
     for (auto const& line : lines) {
         builder.appendff("+{}\n", line);
     }
-    return builder.to_string();
+    return builder.to_deprecated_string();
 }
 };