Make onig crate non-optional

[nathaniel-daniel]

This PR allows the Rust crate to be used without default features.

I ran into trouble trying to add this crate to a project without its default features. Looks like the onig crate has become a non-optional dependency of this crate as this crate will not compile without it. To reflect this, I made onig a required dependency and added a placeholder feature for backwards compatibility.

If adding multiple regex backends like #1510 is desired, this PR should be reworked to detect multiple enabled regex backends and no enabled regex backends and throw a compile error.

[HuggingFaceDocBuilderDev]

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[ArthurZucker]

Thanks for the PR! Actually we'd rather leave the optional dependency and fix the compilation issue