refactor: move _precition_mv_normal_logp into a seperate function

[Michal-Novomestsky]

Addresses #7894 as part of pymc-devs/pymc-extras#532

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📚 Documentation preview 📚: https://pymc--7895.org.readthedocs.build/en/7895/

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[ricardoV94]

Let's duplicate it in pymc-extras first to see if we really need it refactored. In theory you should be able to introduce a PrecisionMVNormal RV and call logp on it to get this implementation. Anyway it will be more obvious if we need the refactor once the dependent work on pymc-extras is ready to be merged.