suggestion: convert single class to standalone functions about capacitor-firebase-authentication

[robingenz]

Hi @mesqueeb,
this should not be a problem in the web implementation.
Unlike the native implementation, which sometimes requires third-party SDKs, the web implementation uses only the popup-based OAuth authentication flow.
The only differences are the different OAuthProvider classes. However, these are very small and negligible.

Because a class can't be tree-shooken I think?

Unfortunately I don't know if class methods are tree shakeable.

Will you consider rewriting these as just stand-alone functions and exporting each function separately instead?

Unfortunately, I don't know if calling these stand-alone functions could still address the native implementation.
Suggested solutions are welcome.
For now, I will not consider this.