fix: add files and remove npmignore (#7500)

# Pull Request

## 📖 Description

The old `.npmignore` was too cumbersome and missed new files going in. This opt-in strategy with the `files` property of `package.json` is a much cleaner and easier to maintain approach for package contents.

## ✅ Checklist

### General

- [ ] I have included a change request file using `$ npm run change`
- [ ] I have added tests for my changes.
- [x] I have tested my changes.
- [ ] I have updated the project documentation to reflect my changes.
- [x] I have read the [CONTRIBUTING](https://github.com/microsoft/fast/blob/main/CONTRIBUTING.md) documentation and followed the [standards](https://github.com/microsoft/fast/blob/main/CODE_OF_CONDUCT.md#our-standards) for this project.

Author: janechu

packages/fast-element/.npmignore

@@ -226,5 +226,9 @@
   "devDependencies": {
     "@microsoft/fast-build": "*",
     "@microsoft/webui": "0.0.10"
-  }
+  },
+  "files": [
+    "./dist",
+    "CHANGELOG.md"
+  ]
 }