update for godot 4

[lstebner]

several things are out of date now that godot 4 is the main version. the source files and README documentation both need updated. it may make sense to do this work in a branch that will live a long time, or to spin off a long-living v3 branch so that both can coexist

some things that are known:

• update Reference type to Object
• update export and onready declarations to @export and @onready