Add a configuration method to indicate that a .env file is optional

[MrDave1999]

This feature will indicate that the existence of the .env file is optional, so it can be in some directory or not.

For example:

new EnvLoader()
      .AddEnvFile(".env.example1", Encoding.UTF8, optional: true) // AddEnvFile(string, Encoding, bool)
      .AddEnvFile(".env.example2", encodingName: "UTF-8", optional: true) // AddEnvFile(string, string, bool)
      .AddEnvFile(".env.example3", optional: true)  // AddEnvFile(string, bool)
      .Load();

In the above example, since the three .env files (.env.example1, .env.example2, .env.example3) are optional, the Load method will not generate an error in case the .env file is not found in a directory.