LibThreading: Make Thread keep itself alive while its action is running

Previously, a `Thread` could be deleted while its action was running,
even if it was running detached.

By changing it to be atomically reference counted, and incrementing the
count when starting the action, we can keep the Thread and its running
action `Function` alive until it exits. Thus, detached `Thread` objects
can be deleted by the thread creating them and allowed to die
naturally.

Userland/Libraries/LibThreading/Thread.h

@@ -8,6 +8,7 @@
 #pragma once
 
 #include <AK/Assertions.h>
+#include <AK/AtomicRefCounted.h>
 #include <AK/DeprecatedString.h>
 #include <AK/DistinctNumeric.h>
 #include <AK/Function.h>
@@ -42,7 +43,7 @@ enum class ThreadState : u8 {
 };
 
 class Thread final
-    : public RefCounted<Thread>
+    : public AtomicRefCounted<Thread>
     , public Weakable<Thread> {
 public:
     static NonnullRefPtr<Thread> construct(Function<intptr_t()> action, StringView thread_name = {})