[SwiftGen-L10n] Improve security when parsing format placeholders

[AliSoftware]

The algorithm for detecting %d and other %… placeholders is not very secure, in the sense that %those chars will fail will be detected as the %f placeholder + the ail string, because it starts parsing the placeholder on % and stop only when detecting one of the known type characters (d,u,i,f,@).

We probably need to stop considering that we are inside a placeholder when we encounter spaces or chars that would be invalid there. Probably check the official grammar for printf too to make that parsing stronger.