Use reflectable package over build_runner for registering of types.

[GameDevBhavesh]

build runner is messy and produce ugly looking code

[fuzzybinary]

I have not used the reflectable package, but can look into it if I get some time, or if you want to do a PoC to show that it would improve the code or the developer experience, I'll happily take a look.

However, a quick glance tells me that reflectable is using build_runner and generated code under the hood anyway, so this would really come down to just how Godot access the properties and trading one set of generated code for another, and by using reflectable I might loose control over some other things I need control over (such as being able to refresh all the type and property definitions on hot reload)

I have significant control over what build_runner generates which is helpful when I need to tag generated code as vm:entry-point (which happens a lot in this code). But, since I do have a lot of control, if there are specific things that are being generated that you think are ugly or (more importantly) inefficient, I can take a look at seeing if I can clean it up.

[fuzzybinary]

Also note augmentations, which will hopefully land in Dart soon, will clean this up A LOT.