Create an openapi dev shell

Move the openapi flake to the root level and refactor to a devshell
called `openapi`.

The other flake.nix files will be similarly combined in future commits.

Signed-off-by: Oliver Davies <oliver@oliverdavies.uk>

flake.nix

@@ -0,0 +1,33 @@
+{
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+
+  outputs =
+    { nixpkgs, self, ... }:
+    let
+      system = "x86_64-linux";
+      pkgs = import nixpkgs { inherit system; };
+    in
+    {
+      packages.${system} = {
+        openapi-generate-html = import ./openapi-generate-html.nix {
+          inherit (pkgs) buildNpmPackage fetchFromGitHub;
+        };
+
+        openapi-mock = import ./openapi-mock.nix {
+          inherit (pkgs) buildGoModule fetchFromGitHub;
+        };
+      };
+
+      devShells.${system} = {
+        openapi = pkgs.mkShell {
+          packages = with pkgs; [
+            openapi-generator-cli
+            openapi-tui
+
+            self.packages.${system}.openapi-generate-html
+            self.packages.${system}.openapi-mock
+          ];
+        };
+      };
+    };
+}