fix(examples/firebase-auth-firestore): Fix imports and throw meaningful errors when missing .env

[penx]

Apply fixes as per comment by @mocon at #1811 (comment)

Also:

• throw meaningul errors when environment variables are missing or can't be parsed.
• check for existence of environment variable rather than cast to string

Closes: issue reported in comment #1811 (comment)

• Docs N/A
• Tests N/A

Testing Strategy:

cd examples/firebase-auth-firestore
yarn
yarn dev
# open http://localhost:3000
# assert "Error: Missing CLIENT_CONFIG environment variable" thrown as env variables are missing
# exit ^C
cp .env.example .env
yarn dev
# open http://localhost:3000
# assert "Error: Failed to parse private key" thrown as example env does not contain valid keys
# exit ^C
# edit .env and set CLIENT_CONFIG to invalid JSON
yarn dev
# open http://localhost:3000
# assert "Error: Invalid CLIENT_CONFIG environment variable"
# exit ^C
# edit .env and set valid keys
yarn dev
# open http://localhost:3000
# assert login page loads

I plan to follow up with a couple of other PRs:

• fallback to Firebase emulator on localhost when environment variables are not set
• use REST API for auth instead of client side SDK as detailed at docs(examples): add Firebase Auth + Firestore example #1811 (comment)
• integrate other Firebase services in to this example as suggested at docs(examples): add Firebase Auth + Firestore example #1811 (comment)

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[penx]

by the way, for examples, it's safe to target the main branch, so you could close this PR and reopen the previous one

closing and opening a third PR as I've rebased so initial PR can't be reopened.

#3353