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

@@ -18,7 +18,8 @@
         };
       };
 
-      devShells.${system}.default = pkgs.mkShell {
+      devShells.${system} = {
+        openapi = pkgs.mkShell {
           packages = with pkgs; [
             openapi-generator-cli
             openapi-tui
@@ -28,4 +29,5 @@
           ];
         };
       };
+    };
 }

openapi/.envrc

@@ -1 +1 @@
-use flake
+use flake ..#openapi