fix(flake): make nix-dev a path input instead of local import

[ohbe1jacobi]

Use a proper flake input declaration:
inputs.nix-dev.url = "path:./nix-dev";

This change prevents downstream consumers from hitting path '/nix/store/...-nix-dev is not valid during flake evaluation. It also preserves the original inputsCombined logic.

Fixes #149

[ohbe1jacobi]

Tested NixOS flake input (nix flake check) via local flake:

[...]
inputs = {
  [...]
  vscode-extensions = {
    url = "path:/home/xxx/Projects/nix-vscode-extensions";
    inputs.nixpkgs.follows = "nixpkgs";
  };
  [...]
};
[...]

Tested NixOS flake.nix input (nix flake check) with github remote flake:

[...]
inputs = {
  [...]
  vscode-extensions = {
    url = "github:ohbe1jacobi/nix-vscode-extensions/nix-dev-flake-path";
    inputs.nixpkgs.follows = "nixpkgs";
  };
  [...]
};
[...]

Both resolved error in #149