Add C language binding for grammar-parser to core.

[philipag]

The main idea is to create a C facade for grammar loading so that it can easily be used to load/allocate (and later free) grammars via any type of language binding.

This facade is extracted from a different project and not yet part of the whisper.cpp build.

This addresses #1930

[philipag]

I now realize that this really belongs in ggml so someone familiar with ggml build files could repurpose this code to there...

However, this also needs to be exposed via whisper.cpp so that whisper clients which don't directly deal with ggml can parse grammars (e.g. I am using whisper.net).

Finally, some language bindings will additionally need a free() method.