json: restore default additionalProperties to false, fix some pattern escapes

[ochafik]

Testing more schemas after #7840, I'm getting an alarming rate of hallucinations on property names. The change of default for additionalProperties may have been somewhat principled but it seems bad in practice (@HanClinto you did warn me, should've listened 😅).

Switching back the default of additionalProperties to false (despite the spec defaulting it to true) requires less caveating in the docs (to avoid slower grammars), and produces less hallucinations (otherwise models can be tempted to add props that look much like known properties and just add some chars to them). Also, additionalProperty keys don't currently ensure their newlines are properly escaped 🫣.

This PR makes the following example from #5978 to work well again (needed extra lil fix of pattern escapes to work again):

./llama-cli --grammar-file \                                          
  <( python examples/json_schema_to_grammar.py https://json.schemastore.org/tsconfig.json )  \
  -p "Write a tsconfig.json for a simple project with strict types incremental compiler/build options:" \
  -mu https://huggingface.co/bartowski/Phi-3-medium-128k-instruct-GGUF/resolve/main/Phi-3-medium-128k-instruct-Q8_0.gguf

[HanClinto]

Sounds good, looks good!

To be fair, I didn't know that it would increase hallucinations, but in hindsight this makes sense. Nice work!

BTW, there is trailing whitespace in test-grammar-integration.cpp that is failing the CI -- otherwise this looks great!