Allow more string escapes in JSON schema

[abcdabcd987]

This PR adds the following to the allowlist of string escape characters in JSON schema:

\n
\r
\t
\\
\uHHHH

I think it's quite normal to allow the following JSON outputs:

{
  "multi-line-answers": "hi\nhello\rfoo\t\u4F60\u597D"
}