Load env.fish from direnv

[vlashada]

Is your feature request related to a problem? Please describe.
In the Fish shell, direnv cannot be used to auto-load env.fish when entering a directory, causing manual sourcing each time, which is inefficient.

Describe the solution you'd like
Implement an auto-load feature in direnv for env.fish, similar to how .envrc files are handled, to streamline environment setup in Fish shell.

Describe alternatives you've considered
Current workaround involves custom Fish shell scripts, but they lack integration and ease of use.

Additional context
This enhancement aligns the experience for Fish shell users with those using Bash or Zsh, promoting efficient environment management.