Consider allowing directory to search for in `find_dotenv`

The issue

Currently, find_dotenv takes the cwd or the directory of the python file that called it. This does not allow specifying a custom directory to search for the .env file without temporarily switching the cwd before calling find_dotenv.

Proposed solution

Add a dir: Optional[StrPath] argument and use that instead of inferring if it's not None.

Start at the find_dotenv entry point and review how it currently infers the directory from the cwd or calling Python file. Done means an optional dir argument can provide the directory used to search for .env without changing the cwd.