async-std support for attribute macro

[allan2]

In the new attribute macro, async-std expansion does not work. This is due to differences between how tokio::main expands and async_std::main expands.

Here are some suggestions for those wishing to take this on:

• use cargo-expand
  • check the expansion of modify-tokio and modify-tokio-macro for reference
  • check the expansion of a plain #[async_std::main] call
• you can determine that you're in async-std when asyncness is true and fn_name is main
• don't have async-std as a dependency in dotenvy-macros
• ensure load_and_modify should be called before the async runtime is spawned
• ensure it works with different function signatures, e.g., Result, empty
• add examples like those for tokio

PRs are welcome :)