Enhancement: Investigate `.env` file support

[SupImDos]

Summary

• As raised by @ericchansen in #34, pydantic-argparse should be able to piggy-back off pydantic and support loading environment variables from .env files.
• This can probably be accomplished by defining arguments as shown below:

  class Args(pydantic.BaseSettings):
      my_arg: int = pydantic.Field(description="this is an int")
  
      class Config:
          env_file = ".env"
          env_file_encoding = "utf-8"
  

• See: https://docs.pydantic.dev/latest/usage/settings/#dotenv-env-support
• If this does work, it might be nice to provide a dotenv extra for pydantic-argparse (similar to pydantic) so people can install it with:

  $ pip install pydantic-argparse[dotenv]