consolidate between llguidance and xgrammar

[jakelorocco]

For structured decoding, we utilize llguidance in our regular Mellea code. For intrinsics that utilize granite formatters, we utilize xgrammar. We should see if we can consolidate to llguidance.

The person who takes this task should check in with Fred to see why xgrammar was originally chosen.

[jakelorocco]

The reasoning for using xgrammar was that vllm uses it. We can consolidate as long as as llguidance has the same functionality (which it should).