[Feature]: Support shell.nix files outside of the project directory

[samuela]

Is your feature request related to a problem? Please describe.
I frequently find myself working in a monorepo setup. It's often the case that I have a shell.nix file in the root of the project and then separate subdirectories for different pieces. Whenever I open up one of those subdirectories in vscode, I have no way of activating the root shell.nix.

Describe the solution you'd like
Some mechanism to activate shell.nix files outside of the current project directory. Being able to simply input ../shell.nix in the "Select environment config" prompt would be huge.

Describe alternatives you've considered
symlinking the parent shell.nix file into subdirectories. splintering the shell.nix into many separate files.

Additional context
n/a