[Feature] Enable XGrammar Regex Support

[FredericOdermatt]

Thanks guys for the awesome work!

I've noticed that XGrammar now supports regex strings xgrammar PR

This PR removes the fallback to outlines for regex for VLLM v0 users (backwards compatibility)

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[russellb]

Thanks for the PR! I've got an older one for this here: #13228

I'm going to close this one out as a duplicate.