What problem does this feature solve?

My project relies on multiple .env files to package multiple environments. Multiple .env files make the project root directory very bloated and inconvenient for finding and managing project files.
With this configuration item, I can keep my .env files out of the project root and manage them better.

What does the proposed API look like?

Would like to have a config item like vite's 'envDir' to customize the .env load directory for rsbuild.
https://cn.vitejs.dev/config/shared-options.html#envdir