Accept karma.conf.cjs as configuration file

Currently, [karma looks for these configuration files](https://karma-runner.github.io/6.3/config/configuration-file.html):
- ./karma.conf.js
- ./karma.conf.coffee
- ./karma.conf.ts
- ./.config/karma.conf.js
- ./.config/karma.conf.coffee
- ./.config/karma.conf.ts

I would like to add `karma.conf.cjs` to this list (and maybe `.config/karma.conf.cjs`).

A `.cjs` extension makes sense in project with `type: "module"`.

[Eslint already searches for a .cjs configuration file](https://eslint.org/docs/user-guide/configuring/configuration-files) for example.