Abort commands not running when max processes < N

[gustavohenke]

Problem

In a few issues, this situation came up where commands that haven't started yet will run anyways when they shouldn't even start.
Best examples of this are SIGINT and --kill-others combined with --max-processes that's lower than N (where N = number of commands itself).

For someone who pressed Ctrl + C, it could be annoying that one press isn't sufficient and they have to press it up to N times (see #452).

For --kill-others/--kill-others-on-fail, it, well... just doesn't work (see #433).

Solution

With SIGINT, it's quite clear that commands awaiting their turn should be aborted.

For --kill-others, there was a possible interaction with --restart-tries: should it be respected at all?
But the flag is documented as "restart processes that died", which isn't the case of a command that never started. Therefore I made the decision of also aborting these commands.

Now, aborting the commands is done here by using an AbortController.
It's quite simple: flow controllers make use of it, and concurrently stops further spawning of processes if the signal is already aborted.

Note

AbortController is built into Node 15+, which is a requirement for concurrently 9.0.0 (#448), therefore this PR is a breaking change.

---

Fixes #433
Fixes #452

[coveralls]

coverage: 99.346% (+0.02%) from 99.324%
when pulling 91d6b3f on issues-433-and-452
into bae6fe5 on main.

[gustavohenke]

@paescuj keen to take a look?