Add timeout support for Exec and ExecForEach commands (fixes #225)

[ehsundar]

Problem

Exec and ExecForEach commands can run indefinitely, causing resource leaks and hanging CI/CD pipelines with no way to set timeouts.

Solution

Adds WithTimeout(duration time.Duration) method for both Exec and ExecForEach commands.

// Basic usage
script.NewPipe().WithTimeout(5*time.Second).Exec("sleep 10").Wait()

// ExecForEach with timeout
script.Echo("file1\nfile2").WithTimeout(2*time.Second).ExecForEach("sleep 5").Wait()

Realistic Example: CI/CD Pipeline

// Build with timeout
script.Exec("go build ./...").WithTimeout(2*time.Minute).Wait()

// Test with timeout  
script.Exec("go test ./...").WithTimeout(5*time.Minute).Wait()

// Security scan with timeout
script.Exec("gosec ./...").WithTimeout(1*time.Minute).Wait()

Why This is Better

• Simple API: WithTimeout() follows existing patterns
• No external deps: Built into the library
• Reliable: Uses context.WithTimeout and exec.CommandContext
• Backward compatible: Zero timeout means no timeout

Fixes #225

[bitfield]

Thanks for the PR. What do you think about the comments on #235 225 about letting users inject a context, and also about using WaitDelay on the command? I don't think we've yet reached a consensus about how this feature should work, so I think we need to do that before attempting to implement anything.