Add working Jest & TypeScript config

- Jest now uses ts-jest for transforms
- TypeScript compilation via `lerna run tsc`

package.json

@@ -24,7 +24,8 @@
     "url": "git+https://github.com/actions/toolkit.git"
   },
   "scripts": {
-    "test": "echo \"Error: run tests from root\" && exit 1"
+    "test": "echo \"Error: run tests from root\" && exit 1",
+    "tsc": "tsc"
   },
   "bugs": {
     "url": "https://github.com/actions/toolkit/issues"