Add Process kill and signal APIs

[floitsch]

Summary

• add immediate Process.kill with an optional hard flag
• add explicit blocking Process.kill --wait with optional hard-after-ms escalation
• treat hard-after-ms=0 as hard-only termination followed by waiting for exit
• add Process.signal as the platform-specific escape hatch
• preserve outer deadlines so an enclosing with-timeout can safely abort the blocking overload
• document that Windows termination is always immediate because arbitrary child processes have no catchable SIGTERM equivalent

This is stacked on #98 because the blocking overload relies on cancellation-safe retryable Process.wait. Native signaling error propagation is in toitlang/toit#3197.

Behavior

• process.kill sends SIGTERM on POSIX and terminates immediately on Windows, then returns
• process.kill --hard sends SIGKILL on POSIX and terminates immediately on Windows, then returns
• process.kill --wait sends the normal termination request and waits indefinitely on POSIX until voluntary exit
• process.kill --wait --hard-after-ms=1000 waits up to one second for graceful exit, escalates, and then waits until exit
• process.kill --wait --hard-after-ms=0 hard-terminates and waits until exit
• process.signal value sends a platform-specific signal; Windows supports signal 9

Tests

• toit analyze src/pipe.toit
• toit analyze tests/process_kill_test.toit tests/pipe2_test.toit
• toit run tests/process_kill_test.toit
• toit run tests/pipe2_test.toit
• both tests passed with the rebuilt SDK
• coverage includes immediate soft and hard termination, blocking termination, invalid grace periods, explicit signals, ignored SIGTERM escalation, handled SIGTERM without escalation, and aborting the blocking overload through an outer timeout