Update setup.mdx (testing-library#1058)

Added instruction on jsdom setup as it now isn't shipped with jest as of v28. https://jestjs.io/docs/upgrading-to-jest28#jsdom "If you are using JSDOM test environment, jest-environment-jsdom package now must be installed separately"

Author: abel-rm-rf

docs/svelte-testing-library/setup.mdx

@@ -9,10 +9,10 @@ with any testing framework and runner you're comfortable with.
 
 ## Jest
 
-1. Install Jest
+1. Install Jest & jest-environment-jsdom
 
  ```
- npm install --save-dev jest
+ npm install --save-dev jest jest-environment-jsdom
  ```
 
 2. Add the following to your `package.json`
@@ -42,7 +42,8 @@ with any testing framework and runner you're comfortable with.
  "transform": {
  "^.+\\.svelte$": "svelte-jester"
  },
- "moduleFileExtensions": ["js", "svelte"]
+ "moduleFileExtensions": ["js", "svelte"],
+ "testEnvironment": "jsdom"
  }
  }
  ```