This fixes CodeQL alert
[#16](https://github.com/MagicMirrorOrg/MagicMirror/security/code-scanning/16)
by replacing shell-built git commands with `execFile` + `cwd` in
updatenotification’s `git_helper`.
It also includes a small cleanup in `checkUpdates()` (remove unnecessary
async/Promise wrapper) and updates the related unit tests.
In PR #4072 GitHub Bot complained about an unused var. Instead of just
removing that one, I checked why ESLint hadn't complained about it: We
had disabled the rule for it.
So I enabled rule and resolved the issues that ESLint then detected.
Related to #4073
Since the project's inception, I've missed a clear separation between
default and third-party modules.
This increases complexity within the project (exclude `modules`, but not
`modules/default`), but the mixed use is particularly problematic in
Docker setups.
Therefore, with this pull request, I'm moving the default modules to a
different directory.
~~I've chosen `default/modules`, but I'm not bothered about it;
`defaultmodules` or something similar would work just as well.~~
Changed to `defaultmodules`.
Let me know if there's a majority in favor of this change.
