Fix: sidecar cleanup when only shell-sidecar is enabled

[busyboredom]

What kind of change does this PR introduce?

• [ x] Bugfix

Does this PR introduce a breaking change?

• [ x] No

Checklist

• When resolving issues, they are referenced in the PR's title (e.g fix: remove a typo, closes #___, #___)
• A change file is added if any packages will require a version bump due to this PR per the instructions in the readme.
• I have added a convincing reason for adding this feature, if necessary

Other information

Sidecar processes are not cleaned up properly unless shell-execute is enabled. This is problematic for apps in which only shell-sidecar is needed.

This fix calls crate::api::process::kill_children() when either shell-execute or shell-sidecar are enabled.

[busyboredom]

I didn't make an issue for this 'cause it's such a simple problem/solution, but if you need me to open an issue before this can be merged I will :)

[lucasfernog]

Nice catch, thanks!