Added jsconfig.json to enable type checking

- we're experimenting with using a simple jsconfig file to enable type checking for Ghost + related repos
 - this should allow us all to see where we have typing issues, without requiring everything to be strictly typed or generating type files

jsconfig.json

@@ -0,0 +1,13 @@
+{
+  "include": ["core/**/*.js"],
+  "compilerOptions": {
+     "checkJs": true,
+     "module": "commonjs",
+     "target": "es2018",
+     "moduleResolution": "node"
+  },
+  "exclude": [
+    "core/built",
+    "core/client"
+  ]
+}