chore: remove usage of command groups (#6992)
### Description
Remove usage of the `command-groups` crate in favor of directly working
with `tokio::process::Command`. This PR just inlines what the `spawn`
call on unix systems was doing.
This PR doesn't add any "process group" handling for Windows as we
weren't using `command-groups` Windows specialization at all:
- On Windows the `spawn` call will [actually
generate](https://github.com/watchexec/command-group/blob/main/src/tokio/windows.rs#L27)
a
[JobObject](https://learn.microsoft.com/en-us/windows/win32/procthread/job-objects).
- Immediately after spawning through we called `into_inner` which [drops
all
references](https://github.com/watchexec/command-group/blob/main/src/tokio/child/windows.rs#L55)
to the JobObject (keeping it open though as closing it would kill the
child)
- Our kill implementation on windows doesn't send any signals, but
simply calls `kill` on the child which on windows calls
[TerminateProcess](https://github.com/rust-lang/rust/blob/master/library/std/src/sys/windows/process.rs#L646C34-L646C50),
which is independent of job objects.
Note, this doesn't remove the dependency completely as it is currently
used when spawning the daemon process and in that case we actually use
the Windows implementation from what I can grok.
### Testing Instructions
Existing unit test for killing process groups on Unix systems.
Manual test on Windows (create a new project, `turbo dev`, Ctrl-C,
verify that the two processes get killed)
Closes TURBO-2033
