Add devshell for TIC-80

I haven't used it. I'm just testing to create devshells with my own
overlays.

shells/default.nix

@@ -4,4 +4,5 @@ with pkgs; {
   flatpak = callPackage ./flatpak.nix { };
   hugo = callPackage ./hugo.nix { };
   rust = callPackage ./rust.nix { };
+  tic-80 = callPackage ./tic-80.nix { };
 }

shells/tic-80.nix

@@ -0,0 +1,12 @@
+# The rest of the tools is your preferences (i.e., image editor, text
+# editor). This comes with the development for PRO version to enable
+# development with plain text cartridges.
+{ mkShell, tic-80, imagemagick }:
+
+mkShell {
+  packages = [
+    tic-80
+    tic-80.dev
+    imagemagick
+  ];
+}