Improve TS

[Gabriel Poussif]

To avoid people like me from spending a long time trying to understand why this fails, this line could be changed from
precondition?: Precondition
to
precondition?: Omit<Precondition, 'exists'>

https://github.com/googleapis/nodejs-firestore/blob/319adc3ff6fdcd0ab61766d4eb8301fc8ac5431b/types/firestore.d.ts#L1469C22-L1469C34

[Tom Andersen]

Thank you for the suggestions. I will bring this up for team discussion.