Properly free resource after thread detach

[todo]

When detaching a thread, we should ensure that the associated resources (stack, handle, context, etc...) are properly freed before the Process exits. This can be done by adding the ThreadContext to a global Vec<> of ThreadContext that gets freed when the main thread (or the last thread alive?) exits.

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SunriseOS/libuser/src/threads.rs

Lines 297 to 304 in 7333eaf

	// TODO: Properly free resource after thread detach
	// BODY: When detaching a thread, we should ensure that the associated resources (stack,
	// BODY: handle, context, etc...) are properly freed before the Process exits. This can be
	// BODY: done by adding the ThreadContext to a global Vec<> of ThreadContext that gets freed
	// BODY: when the main thread (or the last thread alive?) exits.
	}
	}

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This issue was generated by todo based on a TODO comment in 7333eaf when #415 was merged. cc @Orycterope.