Revamp environment check for firebase app

[Feiyang1]

1. Proper browser env detection. Fixed Angular with SSR firebase bug #1754
2. Delay Nodejs detection and warning util initializeApp, so people have a chance to suppress it by setting logLevel in @firebase/logger

[hsubox76]

Especially because we're expecting people to have problems with Firebase Lite at first, I feel like we should make these warnings more discoverable, or make loglevel settings more discoverable somehow. I don't want users to have to come file an issue before they can be told to turn up loglevel to see if anything is wrong.

[Feiyang1]

Warning messages are shown by default, so they don't need to do anything unless they want to disable it. Is it your concern?
The FirebaseLite message can't be suppressed because it will execute before people have a chance to set log level. It is desirable because having firebase imported twice is always the wrong thing to do.

[hsubox76]

Oh, sorry, I thought it was the opposite, that warn loglevel had to be turned on. Looks good then.