chore: lock in TypeScript dependency for monocdk (#7159)

Without the TypeScript `devDependency` directly recorded on
`monocdk-experiment`, `yarn build+test` doesn't work.

Why?

* `yarn build+test` runs `npm run build && npm test`.
* `npm run build` builds a tool $PATH that ONLY includes
  the dependencies of the `monocdk-experiment` package itself
  (not the `$ROOT/node_modules/.bin` directory of the monorepo).
* Since `tsc` is not in the `npm run` path, the system `tsc` will
  be used.
* If the system `tsc` is older than `3.8`, the compilation step
  will fail.

packages/monocdk-experiment/package.json

@@ -23,6 +23,7 @@
   },
   "license": "Apache-2.0",
   "devDependencies": {
+    "typescript": "~3.8.3",
     "constructs": "2.0.0",
     "@aws-cdk/alexa-ask": "0.0.0",
     "@aws-cdk/app-delivery": "0.0.0",