jest-dom seems to include dev dependencies in production

- `@testing-library/jest-dom` version: 5.1.1

- `node` version: v13.5.0

- `npm` (or `yarn`) version: 6.13.4



### Relevant code or config:



```

npm install @testing-library/jest-dom

```



### What you did:



Install jest-dom



### What happened:



I expected it to add few kb of size, but it added 45kb to my library. See bundlephobia for an example:



### Reproduction:



https://bundlephobia.com/result?p=@testing-library/jest-dom@5.1.1



### Problem description:



It _seems_ like some development libraries are included in `dependencies` instead of `devDependencies`. As a quick example, the heaviest dependency [according to Bundlephobia](https://bundlephobia.com/result?p=@testing-library/jest-dom@5.1.1) is `pretty-format`:



- It seems to [only be used in `tests/setup-env.js`](https://github.com/testing-library/jest-dom/search?q=pretty-format&unscoped_q=pretty-format).

- It is [defined as a dependency in package.json](https://github.com/testing-library/jest-dom/blob/master/package.json#L47) instead of a devDependency.



### Suggested solution:



I *think* (from the common JS patterns) that pretty-format might be a devDependency instead of a dependency, so I'd suggest moving it to devDependencies. It seems to be an internal dependency for testing jest-dom itself, so it should be defined as a `devDependency` instead to avoid including it in libraries that *wrap* jest-dom (as opposed to use jest-dom). [e.g. it is an issue here](https://github.com/franciscop/react-test/issues/15).

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