Bump flat and mocha

Bumps [flat](https://github.com/hughsk/flat) to 5.0.2 and updates ancestor dependency [mocha](https://github.com/mochajs/mocha). These dependencies need to be updated together.


Updates `flat` from 4.1.0 to 5.0.2
- [Release notes](https://github.com/hughsk/flat/releases)
- [Commits](https://github.com/hughsk/flat/compare/4.1.0...5.0.2)

Updates `mocha` from 8.1.3 to 8.4.0
- [Release notes](https://github.com/mochajs/mocha/releases)
- [Changelog](https://github.com/mochajs/mocha/blob/master/CHANGELOG.md)
- [Commits](https://github.com/mochajs/mocha/compare/v8.1.3...v8.4.0)

---
updated-dependencies:
- dependency-name: flat
  dependency-type: indirect
- dependency-name: mocha
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <support@github.com>

tests/Avalonia.DesignerSupport.Tests/Remote/HtmlTransport/webapp/package.json

@@ -16,7 +16,7 @@
     "@types/mocha": "8.0.3",
     "@types/react": "^16.3.14",
     "chai": "^4.2.0",
-    "mocha": "^8.1.3",
+    "mocha": "^8.4.0",
     "moq.ts": "^6.4.0",
     "nyc": "^15.1.0",
     "react": "^16.3.2",