feat: provide real standalone providers

[robertIsaac]

usage before importProvidersFrom(provideFirebaseApp(() => initializeApp(...)))

usage now provideFirebaseApp(() => initializeApp(...))

Checklist

• Issue number for this PR: #nnn (required)
• Docs included?: (yes/no; required for all API/functional changes)
• Test units included?: (yes/no; required)
• In a clean directory, yarn install, yarn test run successfully? (yes/no; required)

Description

Code sample

[robertIsaac]

@jamesdaniels @davideast can you take a look at the PR?

[jamesdaniels]

@robertIsaac appreciated, will take a look shortly

[ginagr]

Hi! Any updates on when this PR can be expected to be merged?