Repository: create-react-app
Issue: Error when trying to run an upgraded CRA 4 Typescript project
Labels: needs triage,issue: bug report

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### Describe the bug

When upgrading a Typescript project from CRA 3.4 to CRA 4.0, you get an error when running `npm run start`:

```
TypeError: Cannot add property noFallthroughCasesInSwitch, object is not extensible
    at verifyTypeScriptSetup ({...}/react-cra4-upgrade/node_modules/react-scripts/scripts/utils/verifyTypeScriptSetup.js:232:45)
    at Object.<anonymous> ({...}/react-cra4-upgrade/node_modules/react-scripts/scripts/start.js:31:1)
```

This appears to happen when scanning the tsconfig.json file and modifying it for suggested changes. Based on the error, it appears the object holding the ts config cannot be changed.

When I manually add the `noFallthroughCasesInSwitch` config item to the ts config it passes.

(Write your answer here.)

### Did you try recovering your dependencies?

Yes

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### Which terms did you search for in User Guide?

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### Environment

  current version of create-react-app: 4.0.0
  running from /Users/elylucas/.nvm/versions/node/v12.13.0/lib/node_modules/create-react-app

  System:
    OS: macOS 10.15.6
    CPU: (16) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
  Binaries:
    Node: 12.13.0 - ~/.nvm/versions/node/v12.13.0/bin/node
    Yarn: 1.22.4 - ~/.nvm/versions/node/v12.13.0/bin/yarn
    npm: 6.14.8 - ~/.nvm/versions/node/v12.13.0/bin/npm
  Browsers:
    Chrome: 86.0.4240.111
    Edge: Not Found
    Firefox: 80.0.1
    Safari: 14.0
  npmPackages:
    react: ^17.0.1 => 17.0.1
    react-dom: ^17.0.1 => 17.0.1
    react-scripts: 4.0.0 => 4.0.0
  npmGlobalPackages:
    create-react-app: 4.0.0

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### Steps to reproduce





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(Write your steps here:)

1. Try creating a Typescript based app with react-scripts 3.4.X (here is a link to such an app: https://github.com/elylucas/react-cra4-ts-upgrade-issue).
2. Upgrade CRA by running `npm i --save-exact react-scripts@4.0.0`
2. Run `npm run start` and see the error.

### Expected behavior

Based on the code in `verifyTypeScriptSetup.js` it looks like the user should see a message saying their tsconfig.json is being updated with the recommenced values, and then the app should start as normal.

### Actual behavior

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Get the following error when trying to start the app:
![image](https://user-images.githubusercontent.com/168135/97051413-61ebda80-153c-11eb-8022-0134fbe29cad.png)


### Reproducible demo

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Link to a Ionic React app built with TS and CRA 3.4:
https://github.com/elylucas/react-cra4-ts-upgrade-issue

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