Added coffeescript to dev dependencies

refs gruntjs/grunt#1675

- Grunt 1.2.0 removed coffeescript as a dependency, opting to leave it
  to the users to install
- we use `grunt-bg-shell`, which is built in coffeescript and therefore
  needs the dependency else `grunt dev` doesn't work
- I don't like needing to add coffeescript but it'll do until we give
  the dev experience a good cleanup

package.json

@@ -137,6 +137,7 @@
   },
   "devDependencies": {
     "@lodder/grunt-postcss": "2.0.4",
+    "coffeescript": "2.5.1",
     "cssnano": "4.1.10",
     "eslint": "7.5.0",
     "eslint-plugin-ghost": "1.5.0",

yarn.lock

@@ -1849,6 +1849,11 @@ code-point-at@^1.0.0:
   resolved "https://registry.yarnpkg.com/code-point-at/-/code-point-at-1.1.0.tgz#0d070b4d043a5bea33a2f1a40e2edb3d9a4ccf77"
   integrity sha1-DQcLTQQ6W+ozovGkDi7bPZpMz3c=
 
+coffeescript@2.5.1:
+  version "2.5.1"
+  resolved "https://registry.yarnpkg.com/coffeescript/-/coffeescript-2.5.1.tgz#b2442a1f2c806139669534a54adc35010559d16a"
+  integrity sha512-J2jRPX0eeFh5VKyVnoLrfVFgLZtnnmp96WQSLAS8OrLm2wtQLcnikYKe1gViJKDH7vucjuhHvBKKBP3rKcD1tQ==
+
 collapse-white-space@^1.0.2:
   version "1.0.5"
   resolved "https://registry.yarnpkg.com/collapse-white-space/-/collapse-white-space-1.0.5.tgz#c2495b699ab1ed380d29a1091e01063e75dbbe3a"