fix: report a null exit code when the process is killed by a signal

[shrmguy (leroyomey)]

Fixes #461.

On Unix, onExit reported exitCode: 0 when the process was terminated by a signal, which reads like a clean exit. child_process reports null in that case, and this change does the same. signal is unchanged.

I used the signal reported by waitpid rather than tracking whether kill() was called, so a process that catches the signal and exits on its own still gets its real exit code.

This widens the exitCode type to number | null, so TypeScript callers that assume a number will need a check. In VS Code that is one line in terminalProcess.ts (this._exitCode = e.exitCode). If you'd rather not change the type in a minor release, tell me and I'll rework it.

Tested on Ubuntu 24.04 with Node 24: npm test passes, and the two exit tests fail without the change in unixTerminal.ts. I haven't run it on macOS.

[shrmguy (leroyomey)]

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