Add support for umbrella projects

Motivation

When working with an umbrella project and performing Envy.reload_config/0 we got an exception because umbrella projects define their configuration at the root path instead of hardcoded config/config.exs.

Proposed Solution

Relies on Mix.Project.config/0 to fetch project metadata and access :config_path. This key is always present even if it is not defined in mix.exs, so it will work on both Elixir applications (umbrella or standalone).

Additional details

• After running mix deps.get, mix.lock was updated automatically.
• Could you suggest an approach to write a unit test for Envy.reload_config/0?