add piston nix pkg for emacs

runtimes/emacs.nix

@@ -0,0 +1,30 @@
+{pkgs, piston, ...}:
+let
+    pkg = pkgs.emacs-nox;
+in piston.mkRuntime {
+    language = "emacs";
+    version = pkg.version;
+
+    aliases = [
+        "el"
+        "elisp"
+    ];
+
+    run = ''
+        ${pkg}/bin/emacs -Q --script "$@"
+    '';
+
+    tests = [
+        (piston.mkTest {
+            files = {
+                "test.el" = ''
+                    (princ "OK")
+                '';
+            };
+            args = [];
+            stdin = "";
+            packages = [];
+            main = "test.el";
+        })
+    ];
+}