Added admin/jsconfig.json to fix vscode decorator warnings

no issue

- vscode started adding warnings to all uses of decorators in Admin which was rather annoying
- added `jsconfig.json` with the necessary compiler options and an exclude list to keep performance happy

ghost/admin/jsconfig.json

@@ -0,0 +1,14 @@
+{
+    "compilerOptions": {
+        "target":"es6",
+        "experimentalDecorators":true
+    },
+    "exclude": [
+        "node_modules",
+        "bower_components",
+        "tmp",
+        "vendor",
+        ".git",
+        "dist"
+    ]
+}