Blacklist control characters in JSON string grammars

[jndiogo]

This llama.cpp PR fixed a random problem that I've observed in the past but never been able to reproduce:

ggml-org/llama.cpp#5888

The problem happens because the string definition in the JSON grammar doesn't exclude emitting raw \n, \r, etc. A model can thus produce a string split into multiple lines, which is not valid JSON.

File llama_cpp/llama_grammar.py needs the string grammar definition fixed in several places:

• At the JSON_GBNF definition and others that have a string rule, like JSON_ARR_GBNF, etc.
• At the string definition inside PRIMITIVE_RULES.

It's a single line change in the above PR.

Regards