String format support

[notpushkin]

Hey, first of all thanks for your wonderful project!

Right now the only way to restrict a format of a string is with pattern, but JSON Schema also supports format and some built-in formats could be pretty useful to support.

For example, email is notoriously hard to parse using a regex, but a fairly clean BNF is defined in RFC 5321 (which JSON Schema uses for the email format) and I think it should be pretty straightforward to port.

Do you think this could be something you could use? I can try and work out a PR if you'd like :-)

[adrienbrault]

Absolutely, good idea.

There's a few formats already implemented in llama cpp json-schema-to-grammar.mjs: https://github.com/ggerganov/llama.cpp/blob/2ee44c9a1865a928ccbbc16a2d7841d7513f31c1/examples/server/public/json-schema-to-grammar.mjs#L245-L252