Support threads in the script debugger

* This implementation adds threads on the side of the client (script debugger).
* Some functions of the debugger are optimized.
* The profile is also now thread safe using atomics.
* The editor can switch between multiple threads when debugging.

This PR adds threaded support for the script language debugger. Every thread has its own thread local data and it will connect to the debugger using multiple thread IDs.
This means that, now, the editor can receive multiple threads entering debug mode at the same time.

doc/classes/Thread.xml

@@ -5,7 +5,6 @@
 	</brief_description>
 	<description>
 		A unit of execution in a process. Can run methods on [Object]s simultaneously. The use of synchronization via [Mutex] or [Semaphore] is advised if working with shared objects.
-		[b]Note:[/b] Breakpoints won't break on code if it's running in a thread. This is a current limitation of the GDScript debugger.
 		[b]Warning:[/b]
 		To ensure proper cleanup without crashes or deadlocks, when a [Thread]'s reference count reaches zero and it is therefore destroyed, the following conditions must be met:
 		- It must not have any [Mutex] objects locked.