Add `_timeout` methods for cmd to execute

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Follow suggestion in https://github.com/matklad/xshell/pull/91#issuecomment-2228324022

At the same time, adding a timeout/deadline method, while preserving blocking API, would be great. I think it's OK to burn a thread just for that:

• When we spawn a timeout-enabled process, we also spawn a timeout thread which also gets a reference to the Child's handle
• That thread blocks in https://doc.rust-lang.org/stable/std/sync/mpsc/struct.Receiver.html#method.recv_timeout
• if timeout elapses, it kills the process, which unblocks the main thread.
• otherwise, it unblocks when the sender side of the channel is blocked (it's important that we don't leave hanging threads around)

Add timeout relative methods: run_timeout / read_timeout / read_stderr_timeout / output_timeout.