Move grammar support out of examples? Unify?

[josharian]

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I'd like to add Go binding support for grammars (for https://github.com/ggerganov/whisper.cpp/issues/1697). It's a bit inconvenient now, because the grammar support is off in an examples directory, and adding stuff from examples to libwhisper.a feels wrong; ditto for the header file.

GBNF seems pretty well established at this point (it is in common in llama.cpp). It'd be nice to make it easier to support.

Could we promote grammar support to core whisper.h/whisper.cpp, similar to llama.cpp, where you simply provide a grammar string?