fix mobx-state-tree-typesript missing babel/core dependency (#25589)

## What
![Screen Shot 2021-05-29 at 12 23 37 PM](https://user-images.githubusercontent.com/4411839/120082909-7d8f0480-c07a-11eb-8d77-f9d8769cfefc.png)
![Screen Shot 2021-05-29 at 12 24 46 PM](https://user-images.githubusercontent.com/4411839/120082913-81228b80-c07a-11eb-86c2-24f12811cd24.png)



## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added

## Feature

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added

## Documentation / Examples

- [x] Make sure the linting passes

examples/with-mobx-state-tree-typescript/package.json

@@ -17,6 +17,7 @@
     "typescript": "^3.0.1"
   },
   "devDependencies": {
+    "@babel/core": "7.14.3",
     "@babel/plugin-proposal-decorators": "^7.3.0",
     "@types/node": "^14.0.23",
     "@types/react": "^16.4.12"