feat(useFirestore): support dependent queries

[kiyopikko]

Description

Enable to use the boolean value to tell a query when it is ready to run.

const userId = ref('')
const docRef = computed(() => !!userId.value && doc(db, 'users', userId.value))
const userData = useFirestore(docRef, null)

userId.value = 'userId' // start querying

Additional context

Fixes #2018 @smakinson

You can avoid invalid segment error of firestore path.

Similar interface of https://github.com/Kong/swrv#dependent-fetching

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What is the purpose of this pull request?

• New Feature
• Documentation update

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• Read the Contributing Guidelines.
• Read the Pull Request Guidelines.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).

[smakinson]

I appreciate this, thank you!

[MitchSRobinson]

Hi there, any news on this PR?? I'm currently blocked from properly using useFirestore without this feature so any updates would be appreciated

[kiyopikko]

I removed unnecessary function interface. it's enough to use computed.