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.

core/io/packet_peer.cpp

@@ -117,6 +117,7 @@ Variant PacketPeer::_bnd_get_var(bool p_allow_objects) {
 Error PacketPeer::_put_packet(const Vector<uint8_t> &p_buffer) {
 	return put_packet_buffer(p_buffer);
 }
+
 Vector<uint8_t> PacketPeer::_get_packet() {
 	Vector<uint8_t> raw;
 	last_get_error = get_packet_buffer(raw);