Expose Script reflection methods to scripting API.

[willnationsdev]

When a user wishes to...

1. write tool scripts for the editor to provide or take advantage of class information associated with a script...
2. design their own customized editor that takes advantage of Godot's Script interface, say, via NativeScript or PluginScript...

...they cannot expose information about the resources' class data without first creating an instance of the underlying class. Not only is this wasteful in terms of memory and speed, but it is also a needless complexity since the functionality to access this information already exists within the engine. It simply isn't exposed.

This PR simply adds some utility methods to Script to convert things to Array/Dictionary format. It then binds those methods to the scripting API.

[Zylann]

Note: this won't list properties generated from get_property_list(), because that definitely requires an instance. In fact, (sadly), some ways to expose properties actually require to use get_property_list() :p Example: https://github.com/Zylann/godot_heightmap_plugin/blob/master/addons/zylann.hterrain/hterrain.gd#L135
Just wanted to precise this in case you expected those kinds of properties to be in results or just the "pre-declared" ones.

[willnationsdev]

No, I was specifically referring to only the properties defined by the script, not associated with the underlying class (which would require the instance). Theoretically though, if I only want information about what the script provides, and I have the resource loaded, then I shouldn't need anything else to get that information.

[akien-mga]

Thanks!