Add flake.nix and use it in opencode's flake

[b0o]

It would be very convenient if we could build opencode and opentui from source with nix. Opencode already has a flake.nix but it uses the version of opentui from npm. I've tried overriding this in an overlay but it's very hacky. First-class flake support would be very nice and make it easier to contribute to the projects versus using the current link-opentui-dev.sh script.

[kommander]

I am not a nix user, neither do I even know what a flake is or how to use it, but I don't mind publishing it as such if that helps. Wouldn't know where to start though.

[b0o]

I might give it a shot and send a PR. For now, here's how I'm managing building my fork of opencode+opentui from source using a nix overlay: https://github.com/b0o/dotfiles/blob/main/nix/overlays/opencode.nix

It's quite hacky and requires performing surgery on node_modules to get the build working. But it works 😅