Support fo unicode and octal escapes in string literals.

Specs do not mention whether unicode and octal escapes are supported or not. As clojure.edn supports it [1], I've added an explicit mention in the specs. I'm a registered clojure contributor (signed CA).

[1] https://github.com/clojure/clojure/blob/c6756a8bab137128c8119add29a25b0a88509900/src/jvm/clojure/lang/EdnReader.java#L580