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
- [Commits](https://github.com/hughsk/flat/compare/4.1.0...5.0.2)

Updates `mocha` from 8.1.1 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/v8.1.1...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>

package.json

@@ -29,7 +29,7 @@
   "devDependencies": {
     "coffeescript": "^1.12.7",
     "fs-extra": "^4.0.2",
-    "mocha": "^8.1.1",
+    "mocha": "^8.4.0",
     "should": "^13.2.3",
     "sinon": "^4.5.0"
   },