Godot 4.0 branch

[chrisculver]

Hey there,

I started porting this plugin to Godot 4.0 on a branch in my own fork here. Some of the basic nodes like sequence and leaf nodes seem to work as expected. There are almost certainly bugs due to the rewrite of gdscript.

To save anyone interested from digging into the commits the current concerns in rough order of importance

1. There is no more GDScriptFunctionState
2. State variable of BTNode is not longer setget. Leaving it as a setget makes any behavior tree crash at runtime.
3. Cannot have a signal and function named tick in BTNode.
4. Ongoing bug with onready with new gdscript breaks some decorator nodes. They have been removed from the plugin for now.

[nickpolet]

Awesome to see the start of this being ported over to Godot 4.

Honestly one of the best addons for it. Good luck with porting it over, hopefully the constant slight changes in the codebase don't cause too much hassle.