Style: Enforce separation line between function definitions

I couldn't find a tool that enforces it, so I went the manual route:
```
find -name "thirdparty" -prune \
  -o -name "*.cpp" -o -name "*.h" -o -name "*.m" -o -name "*.mm" \
  -o -name "*.glsl" > files
perl -0777 -pi -e 's/\n}\n([^#])/\n}\n\n\1/g' $(cat files)
misc/scripts/fix_style.sh -c
```

This adds a newline after all `}` on the first column, unless they
are followed by `#` (typically `#endif`). This leads to having lots
of places with two lines between function/class definitions, but
clang-format then fixes it as we enforce max one line of separation.

This doesn't fix potential occurrences of function definitions which
are indented (e.g. for a helper class defined in a .cpp), but it's
better than nothing. Also can't be made to run easily on CI/hooks so
we'll have to be careful with new code.

Part of #33027.

drivers/unix/thread_posix.cpp

@@ -87,6 +87,7 @@ Thread *ThreadPosix::create_func_posix(ThreadCreateCallback p_callback, void *p_
 
 	return tr;
 }
+
 Thread::ID ThreadPosix::get_thread_id_func_posix() {
 	void *value = pthread_getspecific(thread_id_key);
 
@@ -97,6 +98,7 @@ Thread::ID ThreadPosix::get_thread_id_func_posix() {
 	pthread_setspecific(thread_id_key, (void *)memnew(ID(new_id)));
 	return new_id;
 }
+
 void ThreadPosix::wait_to_finish_func_posix(Thread *p_thread) {
 	ThreadPosix *tp = static_cast<ThreadPosix *>(p_thread);
 	ERR_FAIL_COND(!tp);